A while back, we experimented with MPEG 4 in Vegas, and while it was slow rendering, it ran the codecs installed. That was back in the day of Rattlesnake. I don't even know if they are still around.
QT's MPEG 4 codecs show up fine in Vegas, but there is little benefit to them for editing.

I think there are now some cameras that record mpeg 4 to memory stick or similar. They are mighty tiny cameras and I suppose the idea is that the resulting video is small enough to email off to mum and dad while you're on holidays. I doubt the makers of the cameras ever seriously intended you to edit the stuff though. I think the res and frame rate is so low on anything other tha postage stamp size window it's going to look rather sad.

Still though no doubt this is going to become more widespread. Fast silicon is always getting cheaper so comlex compression systems can run in realtime at lower prices. That's why I don't quite get the BluRay concept, maybe when it was first on the drawing board it made sense but I just don't see the need for it, at least not at the consummer level.