I guess I'm in the minority, but I really don't see it as an issue if done well. I have no problem if I see a Coke can lying on the ground instead of one that says Cola. Or a garbage truck with WM (Waste Management) on the side instead of Garbage Company X.
The majority of FPS's are quite dull and bland looking, every Quake is all a shade of brown. No Tampon company is going to pay money to place an ad in Quake Wars, thats just silly talk.
I pay for cable TV, yet I still have ads. I pay for cable internet, yet still get ads. I'd rather not have ads anywhere, but it's not that big a deal, there are plenty of more important issues.