I've been toying around with the idea of wilderness levels for awhile now, and with the new Borderlands campaign I finally decided to try it out. Before I've had a house rule "no hex clearing until level 6 or 7" because of the wilderness dangers, which was added after lots of deaths and dismemberment in previous campaigns. Now it's just not needed. Wilderness isn't safe (it never should be), but now at least it's not so incidentally brutal and randomly deadly either and I personally think it's a good thing because it made wilderness adventures more accessible even to low-level players.
So, the idea itself is simple. Dungeon monsters become stronger as you go deeper, so players can choose their difficulty and reward level by deciding if they want to go deeper or stay on the current level (or get back one level if it's too hard). We can do the same with wilderness, but instead of actual physical levels it depends on the distance from the civilization border in 6-mile hexes. I did it in 3-hexes increment, so level 1 is 1-3 hexes from the border, level 2 is 4-6 hexes, level 3 is 7-9, level 4 is 10-12, level 5 is 13-15, and everything 16+ hexes from the border is level 6.
Now, table "Wandering Monster Table Guidelines" in ACKS (pg.243) shows suggested XP value for monsters in that dungeon level, which works just as well for wilderness levels. Sadly I can't share my tables with you, because it's not tables per se, it's a program I wrote which rolls everything I need, it's ugly and it doesn't use actual tables but rather generates them on the fly and I'm tweaking it all the time, but making your own tables is better anyway. You'd probably have different ideas about monster distribution than I have, especially because some levels for some terrains are very sparse or even completely empty, so it requires some creative tinkering.
Several things to keep in mind.
First, I think the best way is to slowly add stronger monsters to the mix instead of only using stronger monster, otherwise it'd be way too boring, and add a small chance of encountering something usually way too dangerous for this wilderness level. Personally I do it by automatically creating a huge internal table for every level/terrain combination with monster weights based on monster type, XP, and chance of encountering in base game tables, but I'm still not completely happy with the result, so I'd be glad to hear what others do with it.
Second, humanoid villages have a huge swinginess in diffuculty depending on how many warbands you rolled, so you have to adjust that as well (so only 1 warband in level 1 wilderness, and close to max at level 6). I also change group size for non-humanoid monsters a bit depending on the monster/wilderness level difference, so if players encounter griffons, nominally level 4 monsters, right after leaving the civilization, it won't be 2d8 griffons but rather 1d4.
Third, monsters that can have different HDs will occupy different wilderness levels, dragons, hydra, remorhaz, etc...just be careful with your tables so distribution stays relatively same, or you'd have too many dragon spawns in level 2 wilderness for example.
So far it was pretty interesting in play, with definite West Marches vibes. We finished Sinister Stone, and given that Secrets of Nethercity are still not out I had to improvise and decided to try it out. Right now players are about 2-3 levels each but they're already clearing random hexes and making quick forays into deeper wilderness looking for juicy targets with nice loot, yet avoiding anything too dangerous or without treasure. What I really liked was a string of 3 kobold villages pointing to a hex with a very young dragon lair. Dragon was 9 hexes away from the border in the hex with a static burial site with a dungeon, it was completely random but very appropriate. Also players decided that goblins-goblins-hobgoblins-bugbears on the way to another static dungeon means that there must be a goblinoid nation, and now I just have to indulge them (by upping goblinoid frequency in the generator and creating a goblinoid domain 20 hexes away). Can't wait to see what happens when they go deeper and what they would find there.