Mech is still king. The question that comes at you is whether you want to be in a transport when they reach your lines. The answer is always yes. Its best to lose that transport, get out and rapid fire while maintaining as much cover as possible. Lots of mech, and lots of shooting will still severely hurt the incoming Dark Eldar player. Light and Medium armor values don't give lances any extra ability. Don't fear using heavy armor (let everyone else freak out). Predators are still more likely to be hit from the side armor than be hit from the front. Land Raiders are the primary danger here. As I normally dont even shoot at Land Raiders til they get within melta gun and bomb range. Dark Eldar will change that, and Land Raiders will most likely be shot up pretty quickly against all the lance weaponry.
Autocannons with IG and Assault Cannons with Marines. These are going to be a Dark Eldar players worst nightmare. You have FNP tokens... so what. Strength 6 and 7 weaponry double your toughness, so just kill you outright. Guess what, they also break any armor Dark Eldar have pretty reliably. Also it is very likely that these weapons are already in your lists, since breaking light armor is something the guns do quite well, and armies tend to be mech heavy these days. Personally, I prefer the autocannon. These are easy to come by in IG lists, and only somewhat in Marines. They however provide a long range support, that can catch these fast moving armies over a wide area. The Autocannon with its long range reduces the armies advantage of speed. High strength and multiple shots reduces its light armor to wreckage, and is able to insta kill whatever they hit that is holding FNP. Everything else, simply use a bolter and las guns.
Multi-lasers. Such a simple weapon, that we don't count on to do anything, yet here it will shine along with the autocannon. It breaks their armor, it kills their FNP, it shoots a lot, and we have a lot of it.
Heavy Bolters. I've seen lots of people talking about how to deal with these new threats, and most incorrectly are telling us to get out those heavy bolters. The truth of the matter is that the heavy bolter will once again not do a whole lot here. Most armies will be working the system to get those shiney new FNP tokens, and your heavy bolters don't mean anything to a unit that has FNP. Strength 5 doesn't even break a raiders armor reliably enough. Don't go out of your way to add in heavy bolters but leave them on your dakka preds.
Melta Weapons. Wow, no real point in these. I suppose they will blow up vehicles you can get close to, but talk about over kill. (if you can close that distance before they do).
Assault. Here is where you will really have to outplay your opponent. Marine Assault Terminators are still king (blood angels termies 'King of Kings'). However, you will need to make sure that you really tactically play well. You will need to know what can do what, as you head into assaulting this new threat. Know the ranges that Dark Eldar are coming from and can reach with their assaults. Don't forget they are open topped and can assault. They are also fleet. So once you get out to assault someone, you could very easily be counter assaulted from all over the board. Dark Eldar will have some of the most viscous assault units in the game. Using your assault specialists wisely will be key to beating the dark eldar.
Hydra Flak Tanks will be of special use against the Dark Eldar. No flat out skimmer cover saves, no FNP. The gun is twin linked, with huge range, so no need to chase down those fast vehicles. These will be fun.
Range If you can't catch them, you must be able to have the range. Good placement and range will be necessary so that you can keep the pressure on the enemy. If there is anything the Dark Eldar cannot hold up to, is continuous pressure. They are a fragile and costly army.
Above everything. Play your game. Don't let the unique weapons of the xenos terrorize your battle plan. These weapons of the imperium, such as autocannons, fast moving assault cannons, etc, should already be in your lists, so shouldn't change our lists all that much. What you should do though, is learn the capabilities of the Dark Eldar, so there are less surprises on the field.
There are other ways to counter the Dark Eldar, if you have something to add please comment.