Fair use is definitely real, but there's no hard and fast rule on what qualifies. US courts have on one occasion held that even a 3 second sample from a pop song was copyright infringement (the Biz Markie case
), and Notorious B.I.G. had to pay $4 million for having used a 6 second sample without clearance (Bridgeport v. Combs
). In both cases the clip was significantly less than 10% of the song, obviously.
The problem with claiming fair use (apart from the fact that you're far from guaranteed to win) is that you have to go to court to make the defense, which for most people is prohibitively expensive in lawyers fees. Therefore, it's more a question of whether the rights holders (or someone claiming to hold the rights, anyway) will go after you.
A trailer for a commercial game would probably be seen as a commercial, and since licensing songs for commercials is a huge business (potentially the most important source of income for some bands--Biz Markie probably used the money he got from Heineken to pay off his attorneys), they are unlikely to tolerate it... if they ever look at it, which they probably won't.