I was wrong on my previous post. All you need is the magical URL:
http://m.youtube.com/?client=ytdemo

This URL and YouTube mobile video works great on my N93. On my N80i, Real Player says "unable to use network. Phone is currently in offline mode. I have no SIM in my N80i but it should work over WiFi methinks. Bug I guess!

FROM Ring Nokia: Youtube's mobile site is live!:

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Summary: Point you're browser to http://m.youtube.com/?client=ytdemo

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