Creative Mac has an interview with Frank Casanova on Quicktime 6 and MPEG4. Of note, it appears Quicktime 6 will incorporate the MPEG-4 standard:
NAGEL: Everybody's talking about MPEG-4 right now, but what's going on with that?
CASANOVA: ... QuickTime 5 does not have anything to do with MPEG-4. So right now, our version of QuickTime that's being distributed--a million copies every few days--is massively popular, but it remains separate from our MPEG-4 development. We have, though, seeded an early version of QuickTime that has MPEG-4 built in to a set of key developers. And they've had this code for upwards of a month now. This software is letting them start testing the next release of QuickTime with MPEG-4 support and see how it fits into their applications and make sure everything works just fine.
Of note, Casanova hints at a new standard that may overtake MP3:
And the AAC audio component for music will likely replace MP3 as the default and brand new audio standard on the Web because I'll tell you what, ... you can do incredibly good jobs with audio at a much smaller file size and lower data rate and get even better sounding quality than MP3 is providing. I think, over time, we'll see AAC supplant MP3 as the digital audio standard. That's the direction we're headed.