Mono.Upnp is a fully managed UPnP implementation. The client and server stacks are in separate libraries to give embedded devices a compact solution for either. The client library is a solid implementation of the UPnP standard designed to robustly cope with other, less compliant UPnP devices (seriously, no one actually implements the UPnP standard correctly. Especially not router manufacturers, may they die a thousand deaths). The client API is also highly extensible, allowing easy consumption of services which offer functionality not established in the spec. The server library offers developers an easy way to expose their services over UPnP: attributes indicate UPnP-visible members and the library handles all of the network plumbing. In addition, all of the official UPnP device and service APIs (the specs which sit on top of the UPnP architecture - for example, the MediaServer API) are wrapped both for client and server use.
Indeed. Well, almost. All of the above sounds great, but it's not done yet. In fact, it's still quite experimental. While I've made great progress in impressive screenshots, actually listening to your Banshee collection on a PS3 is still a few hack-o-thons off. But things are moving along nicely and I will certainly keep you up to date.
In the mean time, curious monkeys can find the source in the mono-upnp module of the Mono SVN. Like I said, you shouldn't be using this code, but you're welcome to check it out.