Netflix is already streamed to several Linux devices, so the technical issues of a Linux implementation have been figured out. TiVO runs Linux, so does Boxee, and they support Netflix streaming. I expect there are several other embedded devices in the same boat.

The real reason that Netflix doesn't offer desktop Linux support is, as far as I've heard from articles I'm too lazy to look back up to cite, Microsoft refuses to grant Netflix a license to the DRM for a Linux desktop client because Microsoft is concerned that a Linux desktop client would make it too easy to reverse-engineer the DRM scheme that Netflix is using (Microsoft PlayReady).