I have created large (900x516) h264 quicktime movies for use in a training DVD-ROM. The client needs them to play in the Flash 9 player. I changed the extension to flv and then play them in a Flash program and they look fine except there is an occasional random, very short stutter.
They are at a high data rate (6000kbps) but it isn't a disc access problem, I get the same result playing off my 200MB/sec video RAID. Playing in Quicktime they are totally smooth.