Well, good for them, because Snow Leopard doesn't support PowerPC either.
I was naturally referring to classic Mac's there. Modern Mac's should be pretty much the same as x86 except that they apparently have some validation hardware that allows them to run OSX as is. (or so I've been told)
Any modern Linux or Windows should run fine on them.
Michael, apparently Snow Leopard includes some hardware acceleration for H264 videos, so if you could get some graphs showing processor usage between 10.5 and 10.6 that would be great.
Is it exposed through the CoreVideo interface and not anything specific to QuickTime X? If so, it's already covered in the tests, as there is video-cpu-usage monitoring already in there with the CoreVideo output adapter through MPlayer.