I know this might make many of you cringe and post three walls of text about Flash not being fully supported and the pi not having enough horsepower to run youtube's bloated flash player... But before that, I'll just say HTML5. Could it work?
I know that running a full-fletched browser (e.g. Firefox) might be a bit much for the Pi, but afaik not many of the "lite" browsers support embedded HTML5. Hm...
Also, there are (at least for pc/x86 arch) standalone youtube players - are there any that are ported to arm?
So the big question after these disclaimers etc: Is there going to be a way to watch (HD?) youtube videos? Again, not worrying about the "standard" youtube desktop view with Adobe Flash and all, but a decent way to watch streamed youtube videos.
(Hopefully that was enough disclaimers.)
And hey - if there are multiple ways of accomplishing this, the possibilities could be listed in this thread anyway, I think this is something that *quite* many users would like to know about anyway.