Firstly, great work! I love that UKMP is turning in to a fully fledged media player, and I am really impressed with the YouTube functionality.

I've played a .flv from the video part of the media player no problem, but selecting a feature length xvid AVI which plays in mplayer just caused the system to freeze and I had to take the battery out. I haven't had a chance to investigate further but if I can be of help in the testing /debugging I'd be happy to help!