No, not yet.
A friend asks me this question every couple of weeks. Like me, he has an HDTV, but he uses Media Center, so he can’t watch HDTV.
Technically, Vista Media Center does support HDTV. However, there’s no hardware for it yet. When it does come out, you’ll only be able to watch HDTV if you buy a new computer with CableCard support. At the current time, there are no plans to provide a way to upgrade existing computers for CableCard, and no plans to allow you to build a custom computer that supports CableCard.
I don’t know of any hacks for CableCards just yet, but you can hack some cable boxes to get unencrypted HDTV (usually just your local channels). Head over to TheGreenButton forums for more information.