In a sense, it already is. Apple's iTunes uses it as "m4a" files. But what exactly is it?
Mp4 is MPEG Part 14, an ISO standard for multimedia compression, authoring, and delivery. Released in 2000, it is steadily catching on as a preferred format for audio (based on AAC) and video. It uses the *.mp4 file extension, although numerous other extensions that support the standard are available - such as *.m4a (audio), *.m4b (podcast, audio books), *.3g* (3G mobile devices).
The mp4 standard was originally based on Apple's Quicktime format, but was later standardized to make it more versatile and non-proprietary. Microsoft still doesn't fully support it in their Media Player application (except for video, for which you must use 3rd party codecs e.g. Ligos LSX-MPEG or EnvivioTV). The big attraction with this format is how much more it can handle - 3D objects and sprites, for example. It can also be better streamed over the internet.
But why has adoption of this format been slow? Content restrictions (copyright/DRM issues) are largely to blame. But also, although it is an open standard, manufacturers of devices and software must pay license fees because mp4 is a patented technology.
This image shows the layers of an mp4 file. I've noticed in the past that files downloaded from iTunes embedded personal information in the music file, such that if you sent the file to someone, they'd be prompted to enter your password in order to play it. Well, all that information is stored in the Intellectual Property Management and Protection layer. Software that rips music knows how to manipulate this layer, and can successfully wipe the data embedded there to make music playable anywhere. In this day and age, it sucks that when you buy music from the iTunes store, it only plays in iTunes and the iPod. I think we should be more liberal to play music we have bought anywhere so that if iTunes sucks, I can move my music to the Zune and be happy.