Yes, but these 'browser extensions' aren't extensions in the same sense that IE's were. -mox, -ms, and -webkit are vendor prefixes, and they're part of the process for a vendor to get something put into the standard.
That is correct. But until they're actually put into the standard they shouldn't be labeled HTML5, unless you want the term HTML5 to become meaningless.
See this great comment from yesterday: http://news.ycombinator.com/item?id=1403749