It is funny that they appear to be releasing it for MacOS X and using an M$ product to do it. The press release is a little unclear in that it mentions PCs and Macs, but appears to indicate the initial roll-out will be Mac only. My guess is that it's to control the number of users while they test it (fewer Macs = fewer users) plus, Macs have a more consistent hardware/software/OS configuration, so you also have a more controlled environment, better for initial testing/debugging.
Note to Netflix' Mac users, you're about to become a test subject.
Of course, the real "fun" begins when they do release the Windows version. How many different hardware/OS/driver/browser configurations are there?
Why does Netflix insist on tying their online viewing to M$ products? First IE/Win, now Silverlight.
Silverlight, just say no!