According to YouTube Help Center:
What video file formats can I upload?
YouTube accepts video files from most digital cameras, camcorders, and cell phones in the .WMV, .AVI, .MOV, and .MPG file formats.
So how about Ogg Theora?
In a moment of craziness I recorded a very short and excessively lame clip with Istanbul and uploaded it to se what it happens, the end result was unexpected: it worked.
For those reading my post via various Planets where the Flash object is not included and wanting a proof,a here is a direct link but you will not find anything interesting at its end. I think I will start doing more screencasts (if my underpowered desktop will let me record long clips).