Is it a coincidence that Netflix Streaming stopped working in Windows Media Center after the Daylight Saving Time clocks were rolled back? (November 6th 2011).
I called Netflix Support and they said it would need to escalate to Microsoft. They were right. Netflix played just fine in Chrome and IE, just not Windows Media Center. Just in case, I re-installed everything related to Netflix or Silverlight to no avail.
Fortunately, I was able to fix it. This is a really simple fix: Clearing my Internet Explorer cache fixed this on my Win7 x64 machine. I don't use IE for browsing, so I simply deleted all of my Internet Explorer Temporary Internet files. Just doing the cookies would probably also do the trick.
Now, Netflix works again!! I gave Netflix good feedback because it really truly wasn't a problem with their service. Hope this helps others!