There are probably a dozen ways (Kinoma, CorePlayer/TCPMP, etc.), but Google has a free YouTube app for Windows Mobile -- or did. The url for it,
http://m.youtube.com/app gets a 404 error, even though I downloaded the .cab from it a couple of days ago. It's still available on one of the official HTC wikis, but I've posted here.
I won't have the 6900 I purchased from eBay for another day or two, but it works great on the Treo Pro, so it so look even better in QVGA landscape mode.