YouTube Hypes HD

I wish they'd up the length of clip you can upload (still limited to 10 minutes) but it's cool that they are YouTube is now supporting HD: YouTube pushes HD video, new landing pages

They have a page with details on optimizing video.

Resolution Recommended: 1290 x 720 (16x9 HD) and 640 x 480 (4:3 SD)
Bit rate: No recommended or minimum value
Frame rate: The frame rate of the original video should be maintained without re-sampling. Pulldown and other frame rate re-sampling techniques are strongly discouraged.
Codec: H.264, MPEG-2 or MPEG-4 preferred.
Audio Codec: MP3 or AAC preferred
Sampling rate: 44.1kHz
Channels 2: Stereo