YouTube launches video player for Windows Mobile, Nokia S60
You can use the application to search or browse for videos and play them on your device. When you click play, videos will launch in full screen mode.
To install the application, visit m.youtube.com on your mobile device and click the download link. There's a list of supported S60 devices, but no support list for Windows Mobile. Having tried unsuccessfully to install the YouTube player on my Dell Axim X50v, I can tell you that it does not support Windows Mobile 2003SE. But I suspect if you've ot a device with Windows Mobile 6.0 or a newer operating system you should be covered.