m.youtube.com offers mobile optimized content to 3G or WiFi enabled handsets that support RTSP streaming. The mobile page supports logins and channel subscriptions, and is available in country specific form as well.
YouTube has also released a beta version of the YouTube for Mobile downloadable application. The app supports the Sony Ericsson k800, w880 and the Nokia e65, n95, n73, 6110 navigator and 6120 classic, providing the same YouTube browsing functionality currently available on the iPhone.
To obtain the app, users on compatible phones should visit m.youtube.com, where they’ll be asked if they want to install it.
More details on the YouTube blog here.