If QuickTime can play a certain audio file type, iTunes can also. However, you'll find that it will lagg a fair bit (you'll hear the audio stutter when hiding and showing the app) plus other features such as visualizations and equalizer will not work.
iTunes 4 is widely rumored to feature full MPEG4/AAC support as well as Rendezvous sharing and Smart Visualizations.
Dude I thought this was old news. I played mp4s in iTunes back when Quicktime 6 came out. I thought the 'mp4 support' the future iTunes is rumored to have was so people could convert the CDs directly to mp4 from iTunes instead of going through Quicktime.