Please note that Plex no longer supports a Netflix channel (thus the following only has historical interest).
You could also use the Plex Media Server (PMS) - unlike PlayOn, it's free. As far as I know, the server needs to be able to play Netflix though. I use Windows 7 as the Plex Media Server. I don't know if using the Netflix-on-Ubuntu workaround will get this done as well, but I doubt it. There are PMS builds for Linux.
You will need to install the Netflix Addon on the PMS and the Plexbmc Addon on the Raspberry Pi.
In the Plexbmc options, set "always transcode" to true.
This worked on my setup with OpenELEC on a B-model Raspberry Pi - it should work with any XBMC-on-Raspberry-Pi setup though.
Of course, this still means a second server running Windows 7 needs to be on for streaming and currently the Netflix Addon for PMS only works in the USA.