YouTube Cookies and Gnash
The 0.8.8 release of Gnash handles this correctly, but the user needs to delete all their YouTube cookies first and restart with a fresh one. That's all! For older browsers, you also have to block the YouTube cookies after deleting them to get this to work.
For me in all my testing since fixing the cookie problems many months ago, 100% of all the YouTube videos I've tried work, HD included. It appears others are not having the same high success rate as I did. Since multiple requests for pre-release testing were ignored by most everyone, your mileage may vary...
If people want better free Flash support for online video, we'd appreciate funding help, and all the rants about how F&$%@#%d up Gnash is don't really do us any good, since we're all working for free...