Apple recently released a public preview of QuickTime 6
It is able to do MPEG4 now, but there are licensing issues still in the works. My concern isn't that MPEG4 isn't a good technology, because it is. I used MPEG4 codec when I code my video now, the problem in my mind is the streaming or MPEG4 video. The linux desktop doesn't have any easy way of bringing this video straight to the desktop. I currently have Quicktime 5 running through Wine, but I don't know if Quicktime 6 will work. I will give updates when available.