Equate this to movie product placement...
I'd have *no* problem with adverts in videogames, as long as they were treated just like major motion picture releases. Just like most people have already stated, it would be just fine to see an advert for say Motorola in "GTA IV", but not in "BF 1942"... just like it would be fine on to see the same ad on the silver screen in say "The Happening" but not "300".
It's all dependent on context, which is the way Hollywood has been doing things for years now anyway.
That said, I will be royally pissed if I start seeing really obtrusive ads in games. I'm already fed up with being subjected to "nVidia, the way it's ment to be played", "Plays best on Alienware", "Intel this, that, and the other" etc. etc. every time I launch a modern PC game.
Let's hope the developers (not the publishers, there's no hope there) realize that there is a real possibility they will alienate their fan base if things get out of hand. However, even with forced ads in game, the amount of people pissed off enough to boycott will never reach critical mass to change anything. That's why we're forced into previews on VHS/DVD, in the theaters, etc.