Since Netflix is the only silverlight content I've encountered so far, I haven't been able to test this plugin yet. Flash appears much more prevalent - and it works
EDIT: I tried other silverlight content (eg. NBC Olympics), but all the websites claim that my system isn't compatible and won't send the streams, even though I have the Moonlight 1.0 plug-in available. Perhaps their filters are flawed and dismiss me on the basis of Linux (Ubuntu 8.10) despite Firefox 3.0.6 and Moonlight 1.0.
FYI, I am aware the confusion isn't Boxee's fault. I'm just hoping support arrives soon by whatever route possible.