YouTube doesn't work on a brand new MacBook
So this isn't a total solution but it should help.

You reported that even though you joined the html5 program the following video did not play in html5:

http://www.youtube.com/watch?v=5NV6Rdv1a3I 

I was able to reproduce that result in Chrome.

Here's how I was able to get the video to play in html5:

In Chrome:

View > Developer > Developer Tools > Click on the little gear icon

In the resulting preference window:

Settings > Overrides > User Agent [X] > iPad -- iOS5

Then reload the video and it will be in HTML5.

Similar solution exists in Safari, but I have to walk the dogs. Back soon.