I was literally just wondering about this in the course of writing about Media Center for Windows 7 Secrets (and I couldn't get any of the third party Netflix plug-ins to work with the Media Center version in Windows 7). From Microsoft:
As part of ongoing efforts to make Windows Media Center the best place to experience TV on the PC, we wanted to give you a heads up on the newest content addition to Windows Media Center: Netflix. Available today are more than 12,000 movies and TV episodes that can be watched instantly on any PC with Windows Media Center, which is included with Windows Vista Home Premium or Ultimate, and you’ll be able to search the entire Netflix library, manage your instant and DVD Queues and even filter searches by titles that are available to watch instantly.
To get started, simply click on the green button under the Start menu, and select the new Netflix tile under TV+Movies.
With the availability of Netflix in Windows Media Center, we’re building on our broader vision to alleviate the need to jump from Web site to Web site to find the TV shows, movies, sports and news that you want to watch. With Windows Media Center, you can now find it in one place - with a consistent experience.You can also check out our latest TV On Your PC blog post that includes some broader discussion points, an animated video that provides an overview of the Netflix application features, and a Man on the Street video that we made which gauges peoples thoughts on how they like to receive their content and why.
More info...
Netflix in Windows Media Center (Microsoft)
TV on Your PC blog post (Microsoft PR)
So it doesn't appear to work in the Windows 7 RC (I'm sure it will be there eventually), but it's up in Windows Vista Home Premium and Ultimate.