First of all, if someone is finding adamantine weapons for cheaper than normal +2 weapons, I want to know where they're buying their adamantine, because everywhere I've gone adamantine +2 weapons cost 1,000gp more than magical +2 weapons.
Also, adamantine weapons do cut through DR, they have an enchancement bonus and an enhancement bonus is required to cut through DR, therefore they cut through DR. End of discussion. (I'm not going to get into that argument again, because I'm right, and there's nothing in the core rules that suggests differently)
The incorporeality thing is a little unclear since it says +1 magical weapon, but I would allow adamantine weapons to hit incorporeal beings, though I suppose it's a little bit more up to interpretation there than in the DR-bypassing issue (which has nothing to do with interpretation, because they bypass DR, and that's all there is to that).
It's interesting that someone brought up the stacking enhancement bonuses thing...if it were true that "natural enhancement bonuses" and "magical enhancemnt bonuses" were indeed different by the rules, then they would stack, as they are different bonuses (which proves again that DR is bypassed by adamantine). Personally, I'd say that you can't add a special ability to a weapon that's made of adamantine, since the special ability is derived from magic, and the adamantine has no magical enchantments on it already, and I always read that part of the rules as meaning that the sword had to already have a magical enchantment on it, not just an enhancement bonus...then again, I've always thought that was a stupid rule anyway, so I would probably allow a +2 adamantine keen sword, but I would charge for a +3 sword +1,000gp, or maybe a little more than that...not really too sure about that one.