"It works" != "It's supported."

"It's supported" means they'll allow people to report issues, and they'll address them. It means they'll test their site on IE, a non-trivial thing thanks to IE being a browser only available on Windows machines. 

I applaud Microsoft's embracing of open-source and open-standards. It's going to make life a lit easier, and I look forward to IE10. That their stuff works on IE shows how much of a commitment Microsoft is making with IE.  But just because it works doesn't mean it won't be supported.

And also, thanks to libraries like Compass, the -khtml prefix is automatically inserted. So the library they use for their styles may just be giving that away for free.