On the contrary, the Mono Project actually has an equivalent, called Moonlight, which I believe is more-or-less sanctioned by Microsoft in some fashion, although it is Open Source. I'm not sure of the details of the "sanctioning", or whatever you want to call it, but given that Silverlight was an attempt to kill Flash that kind of fizzled (the only users of it I know are those like the OP who need it for a few streaming video sites that MS has probably payed to use it), cooperation wouldn't surprise me. For that matter, if I were them, it would make perfect sense - essentially free labour to bring it (well, a reimplementation) to another platform, and at that, a platform that already hates Flash because Flash has no complete Open Source implementation.
Back on the original topic, though, to get Moonlight on the N900, you'd need a good portion of Mono as well, and I don't know if that's practical. At any rate, it hasn't happened yet. One thought - and this is a long shot - is that since there is an Open Source implementation of Silverlight, it might be possible to transcode-on-the-fly whatever Silverlight's analog of Flash Video (flv) is on a desktop machine, and re-stream it to your phone. I have no idea if this has been done, if it's even possible, or if it would be an acceptable solution for the OP, but it's what came to mind. If this is possible, and anyone does it, keep in mind copyright implications if you intend to stream it outside of a LAN.