I've recently stumbled upon an article on http://www.webmonkey.com/ (an old favorite) that talks about HTML 5. It actually looks pretty fun, too.
Anyway, I was wondering when would be a good time, in terms of compatibility with browsers etc, to begin writing HTML 5 web pages?
Now. And, yes, I'm using it, as are a number of developers. As long as you don't get too exotic and delve into the gray areas, you should have few problems. By gray areas I mean stick to the new standard elements at first: nav, section,aside, header, etc. Branch out after you become comfortable but do not fear it.
I've not started yet, but there's no reason not to. Some useful links: