My understanding is that youtubeplay has died (though it had been working wonderfully since shortly after my 2-yr old post above, until just recently). Next best alternative is mVu, though it requires a separate player (no problem, TCPMP works well for me), and then there's the old WVD (Web Video Downloader). Here's a link for mVu:
http://forum.ppcgeeks.com/showthread.php?t=125286