Hi Alex and Co:
I understand why you need to declare spell casting and retreating before rolling your initiative from a mechanical sense, but I’m unsure why you need to do so for withdrawing. Would you mind explaining it for me :).
It's intended to force an interesting choice on the player. As he watches his hit points dwindle, does he make the proactive decision to flee? I suppose it's more of a psychological mechanism than a mechanic. Players can become quite angst-ridden over the decision to declare a withdrawal.
It’s also important because you have to make an “uninformed” decision. If your attacker misses you, maybe you’d prefer to get in one more attack of your own before you withdraw, but you won’t know until it happens.
You might also think of it as another penalty for breaking from melee. Sure, you can only make a half-move, but declaring withdrawal also draws attention to yourself. I mostly see it with monsters (“He’s withdrawing? What’s he up to? FOCUS FIRE ON THAT GUY!”), but it could easily happen to PCs and henchmen as well.
Okay, got it. I do like the uninformed decision aspect that’s created. I’ve another question then:
Is it intended that the Judge openly declares if a monster casts, withdraws, or retreats as well? I finally got a chance to play a real session with a couple of friends this weekend and what I did was openly declare if an enemy was casting a spell but privately determine if an enemy would withdraw or flee before rolling its initiative.
In my games, I strive to have the enemies and the PCs play by the most similar rules possible, but I don’t see a reason to announce withdrawal so long as you can resist the temptation to change your mind.
By the rules, one declares it, so I declare it.
I guess the question is “what’s the purpose of declaring?”
If it’s just to keep you honest, then I suppose the Judge doesn’t have to say it out loud. However, not declaring it out loud prevents the PCs from trying to cut off the escape, while the PCs have no such luxury when they want to run away. Is that intentional? I don’t know.
Here’s a question, though:
A spell is at risk of interruption as soon as you declare it. Is it the same with retreats? If I declare a retreat, can I be attacked at +2 or back-stabbed before my turn even comes up? Or do the penalties only go into effect once I’ve moved?
I’ve sorta looked at it from this angle:
I assume that casting any spell takes a significant portion of a 10 second combat round, hence I as the Judge, declare when enemies cast spells to “simulate” the time involved and of course to give PCs a chance to disrupt it.
I don’t feel the same way with withdrawing and retreat: I don’t feel such actions are telegraphed the same way as spell casting. Hence, I didn’t declare them openly.
So I’m wondering if it’s intended to be one way or the other, or either way as the case may be… plus it’s always enlightening seeing how other GMs run things. The more I dwell on it, I can see pros to announcing defensive movement for monsters too, as I haven’t given it much thought before.
Also, for me at least, the rules seem pretty clear that once you declare a retreat, you suffer the penalties listed immediately… from a mechanical perspective, that’s the exact reason why you have to declare in the first place… you’re risking taking an attack at +2 etc. and it all comes down to who gets initiative: if you beat your opponent, you’re scott free (sort of), but if you don’t, you’re more likely to eat an attack.
The problem with announcing it is, if the PCs win initiative, they have knowledge of the future. With spellcasting, it should be immediately obvious what’s going on, but withdrawal? I’m nto so sure. Likewise, the enemies that you control have no business ganging up on a PC for no reason other than their announcement if they haven’t yet, right? How would they know that they’ll get the bonus?
But you aren’t seeing the future. Combat is a swirling chaotic mass…initiative is a way keeping track of things for us but characters aren’t really ‘taking turns’. As soon as A decides to withdraw the are beginning to attempt to fight their way out, a type of action that B and C can respond to. Otherwise A can escape without anyone having the chance at grappling or tripping or stopping them in some fashion.
To add to what Tywyll said, consider that if the Withdrawing combatant rolls the worst possible initiative, he can now Withdraw without anyone having a chance to react, making being slow/clumsy/unlucky an advantage, which doesn’t seem right, either.
I’m wary of using the appeal to “real” combat, since it has some slippery-slope potential. I should also clarify that I’m not suggesting the penalties don’t come into effect until your number comes up, so if you lose initiative you will still be standing in melee at a disadvantage while you wait to have withdrawn.
On the other hand, your point about grappling is pretty convincing because of the analogy to interrupting spells.
Just to keep thinking this through: Withdrawal is a full-round, interruptible action (like spell-casting), but a character with the Skirmishing proficiency does not take the AC penalty until their initiative number comes up and can’t be interrupted.
B and C can grapple/trip A by simply trying to do so on their turn in the normal fashion. So they can certainly have a chance to stop A before he withdraws. The difference is that they have to anticipate it rather than react to it with regard to the Judge declaring openly or not to the players.
Assuming that is true, then even a combatant with high initiative can simply hold his initiative until his assailants have acted, and then withdraw.
But I don’t think that is true 
If you have higher initiative than your opponent when Withdrawing, you retain your attack if your opponent decides to pursue (attacking simultaneously with him). If you don’t have higher initiative, your retained attack is wasted.
Also, if you have higher initiative than your opponent when Withdrawing AND you’re in melee with allies, you may withdraw without the enemy pursuing, thus saving you from more attacks. If you don’t have higher initiative, then you may endure more attacks from your opponent before you finally withdraw from the combat.
And also, based on Alex’s response above, declaring a Withdrawal before initiative seems meant to influence the combatant’s proactive decision to do so, rather than to influence his opponent’s reaction to the Withdrawal. The inverse seems to be true in the cases of declaring spellcasting and Retreat.
I think it’s safe to say that it is less likely that a Judge will abuse the “foreknowledge” Players declaring a withdrawal, compared to the other way around… so for the time being, I’ll continue to keep the monster’s withdrawal and retreat declarations quiet. 
There is nothing in the section for a Full Retreat suggesting it is interrupted by damage, only for spellcasting: “If the caster takes damage or fails
a saving throw before he acts, the spell is interrupted and lost.”
As for me, I declare openly when monsters are withdrawing or retreating. I have yet to encounter a situation where I felt the combat had been broken in half by PCs having foreknowledge of a withdrawal. If they were hoping to withdraw as well, they had to declare it as well and can’t change their mind. If they’re still on the offensive, they’ll simply close the distance with the enemy.
The interruption is metaphorical: One of the reactions to knowing someone is withdrawing is a grappling or tripping attack, which would “interrupt” the withdrawal.
Ah, ok, now I see what you mean. It seems, then, that not declaring it is weakening the skirmishing proficiency by making its effects less unique and impactful.
Just to be clear Jard, I still determine if a monster casts, withdraws, or retreats before I roll its initiative… I just don’t announce the latter two options to the players. This should have no impact on the effectiveness of the skirmishing proficiency, which is predominantly an ability within the purview of the players.