Change to a different quality and then it may actually start to buffer and play.
Ive tried disabling the youtube HD extension and letting it play in 480 or 360 and that seems to work a little better.
The stats in the right click, show video info just sits on 0kb/s for ages, like 2-3 mins before moving again. whereas a few weeks ago when I was last using it, it would sit at pretty close to line speed till it had buffered all it wanted to, or else had buffered the whole video.
Anyone else seeing things like this? Sounds a lot like the problems that were coming up in the vodafone youtube thread a couple of weeks back.