Coming Soon: A 3rd party Hulu plugin for Windows Media Center
Zinc and Boxee have both offered solutions. But Boxee is still in Alpha, and right now it's only publicly available for Mac and Linux users, although a Windows version is in private alpha testing. Zinc's application actually works pretty well with Windows, but navigating the list of TV shows with a remote control can be a bit of a hassle. And there's no simple way to integrate Zviewer with Windows Media Center.
Now there's a new player in town, called SecondRun.tv. And it's designed specifically to work with Windows Media Center. The plugin isn't available to the public yet, but the developer says it's fully functional and he's posted a video to prove it. SecondRun.tv should be ready to release soon.
The plugin will let you browse and watch Hulu videos on any machine with Windows Vista Media Center. Odds are it will also work with Windows 7. And while it hasn't been tested with Media Center Extenders like the Xbox 360 yet, there's a pretty good chance they'll work too.