I'm a bit confused that it hasn't been mentioned more, but the problem with MPEG video has existed since 10.0.0 on my machine, and is still there in 10.1--I'm pretty sure that it's related to QT, not the OS itself. At least in my case, though, (and I have a DP533, so I'll admit I'm throwing a heckuva lotta power at it) QT for X handles almost all other (non-MPEG1 mutexed, that is) media just fine--even Sorenson video, which is much more processor intensive, works far better than MPEG. The LotR fullscreen trailer played fine (even dragging) for me, I can play three quarter screen (320X240) Sorenson compressed trailers simultaneously (along with a couple of MP3s) with no dropped frames and no significant slowdown, and I can play at least 15 MP3s simultaneously (yes, it's useless, I was just curious) without any of the tracks in the cacophony skipping, and without maxing out the processors.
Basically, MPEG1 video is the only thing that causes problems--even MPEG2 must be ok, or DVDs (which as poor hardware-decoder owners have discovered are software decoded) wouldn't work well. I'm betting that the people here who don't see any problem haven't messed around with large-frame MPEG...
The one thing I've never been quite clear on is whether this is a universal problem--I've heard several other reports of it (and at least one was from a single processor Mac owner, so it's not just a DP issue), but not as many as I would have expected if it's universal (then again, maybe not that many people use MPEG video now). Has anybody definitely played an MPEG video clip (little ones don't count--at least 320X240) without using up most of the available processor power and slowing QT player to a crawl?
And, of course, if you have, what're you doing differently?