I just read this elsewhere:

"First it's a myth that Youtube converts "everything" you upload.
If you work within certain constraints Youtube will save itself some unnecessary effort and fastforward your video straight past the encode que and just use exactly what you upload.
So no waiting for your video to come online - it appears within a few seconds.
To pass this test your uploaded video should first already be a suitably encoded Flash video file - FLV.
Next - Youtube works on an "average bit rate". So other than getting your video in to Flash format (if not already) that's all it's really bothered about. It wants a smooth playback - avoiding undue buffering.
So - if your uploaded video file is FLV and has an average bit rate below the Youtube target (think it was 300kbs) - you get fasttracked - no encoding!
This opens up an interesting loop hole that can be exploited where you can encode your own FLV at a much higher bit rate than Youtube would normally use - and trick Youtube into allowing that video untouched straight online.
As a test using this technique I encoded a video that has such a high bit rate (700kbs) it can't actually play without loads of buffering on most fast connections (exactly what Youtube is seeking to avoid). No purpose in this other than to prove the video had its bit-rate intact and had by-passed the re-encode que.
You may already see where this is going??
If your video is quite small less than 10-minutes, you can create a false "average" bit rate by padding the end of your video with "black". These black minutes will have a low data rate and thus push down the overall average for your video.
Then encode your own Flash file at a higher bit-rate than YouTube default and then upload your video.
Basically going in under the Youtube radar!"

Is that cool or what?