Cool, quick response!
The guys in our office made their own plugin a while back. We started using yours because they never got around to building support for playlists and queueing videos (they just used player.open).. THOUGH we do have support for Vimeo and College Humour.
Just another thought for you (I think I saw you said you wanted to support other video sites) - to give you some feedback on our experience:
Your UI stuff for YouTube is nice.. BUT to make support easier, have you thought about moving your UI to an extension button beside the URL/Omni bar?
This will save you rewriting your UI for each of the video sites and make for a consistent experience across all video sites.
(This is just a quick hacked up UI we're using)
Again, just thoughts - there's definite merit in integrating with the Youtube UI..
BUT it makes support a biatch for multiple sites. Also if they decide to screw with IDs you're forever updating your code!
Anyway, keep up the good work!