I put a very short video together to test YouTube's new HD capability. Unfortunately, I haven't been able to export a format that works well. The video either doesn't register as HD or the encoding is very choppy.
YouTube HD is working now and can be viewed here. The settings that finally worked were:
5000 kps data rate
optimized for streaming
1466 byte maximum packet size
The embedded Vimeo clip below is low res. Here's the link to the vimeo HD version.