AppleInsider notes that development of Quicktime 6.4 has been completed.
The newest version of Quicktime is said to incorporate 3GPP and multi-channel AAC, and should be incorporated in Panther (Mac OS X 10.3), according to the rumor site.
Quicktime 6.3 was released in June alongside iSync 1.1. Quicktime 6.3 first brought 3GPP support -- which is aimed at mobile audio/video applications