FWIW (I don't have one, so maybe not that much) the impression I've got is that youtube vids work a lot better on the N900 when in fullscreen mode.

I think this is quite visible, for example, in the example given above: http://www.youtube.com/watch?v=ryaO9WHaxlo .

When viewed as part of the youtube page there is definitely stuttering - for example at around 1m 28s in, but there is a very noticeable improvement around 1m 45s when fullscreen mode is used.

My guess would be that the youtube site has lots of other rendering going on, and this degrades the quality of videos seen in "part of the page" mode.

Wish I could try out this theory in practice..! ;-)