Using AAC is brilliant for several reasons. First, for Apple, whose stated market aim is to do everything in its power to sell more of its highly profitable iPods (and beginning in June, presumably profitable iPhones), the choice of AAC means more non-Apple devices will be able to play songs purchased on iTunes.
AAC might become more popular over time, but I’d be surprised if it overtook MP3. That format has just been around far too long to be fazed out anytime soon. Either way, the good thing is that most modern players (even non-iPods) play AAC already.
Source: Business Week