YouTube responds to Hulu with YouTube Everywhere, live video, and TiVo, but also with higher quality video according to the YT blog; see YouTube Videos in High Quality. A low quality version is also transcoded and made available to the low end, and a preference is available on the user's account.
The new spec is 480x360, up to 900 kbit/s A+V (1 GB upload size limit), but getting stereo hasn't been a sure thing. Various players think the hi-qual clips are h.263 or FLV only as well as h.264, and some of them at least won't play in QuickTime Player when relabeled as .mp4.
Wired has a handy summary of some of the work done by YouTubers who watched the transition for the last few months.
In the old system, it seems you could get stereo if you submitted a movie (FLV or other) with a lower data rate than their threshold for resampling, which is 350 kbps. According to a thread on Video Help, "if you use mencoder, this code is simply amazing for stereo audio, and often gives you a lower average than 96 kbit/s (what they use now):
-oac mp3lame -lameopts vbr=2:q=8:aq=0:mode=1:padding=2:lowpassfreq=16300 -channels 2 -srate 44100"