hmm, I was under the impression that people already started to code via HTML5 .. at least that is what I keep hearing. I guess it would be safe to code in HTML5 but use XHTML syntax then when HTML5 is fully supported you can begin implementing the HTML5 specific features. ...
Here's a site that, when I sniffed through the code, was as much HTML5 as they could stand: betribes.com I mean, if you look at youtube.com it's mostly the doctype and they wanted to use <video> (which lets me actually use youtube on my machine as I don't have Flash but I do have Chrome).
Why SitePoint uses XHTML? Because the X is seXy. There is no other reason. The X is seXy. Similarly, many of the sites using HTML5 think the "5" is sexy. Meh.
I prefer HTML4 Strict BUT I will send it through the validator and tell the validator to pretend it's XHTML1 Strict. So I write as strict as I would if I were writing XHTML, but don't use that doctype and don't use closing slashes (if they're ignored by the browser, then why do I have them?). Yeah, same could be said for selected="selected" when the browser only bothers looking at the first "selected", but again I guess it's the strictness that's appealing and /> would invalidate my HTML4 : )