This essentially boils down to getting Netflix on Linux; which (except for Android) is intentionally not supported and difficult. I haven't seen any solution yet (for any Linux, RPi aside) that didn't involve serious and unstable hacks or some type of Windows emulation/re-implementation (which is not going to be a viable option with the ARM/x86 architecture difference).
Your best bet is probably to wait for Android Pi to become stable and hope Netflix runs on it successfully (or hope Netflix finally supports Linux besides Android).
Update Jan 2018:
It's been 5 years and much good news is to be had.
Netflix now supports HTML5-based playback, so Silverlight is no longer a blocker.
The appropriate DRM-decrypting/decoding libraries have been written for Linux and include Raspberry Pi 3 support.
You can probably use Firefox or Chrome at this point just fine, but even better for Kodi users: Kodi 18 (release date to-be-determined) includes a redesign of the video system which has opened up the ability to integrate DRM-ed content into Kodi playback. Presumably Raspbmc will have the changes integrated sometime after Kodi 18 is released.
The end result is that Netflix on RPi should be feasible by the average user of Raspbmc once Kodi 18 is released. One catch however; I think any Linux-based library is still artificially limited to 720p resolution by Netflix, but I'm not sure on that.
See https://github.com/asciidisco/plugin.video.netflix for details and updates.