I was reading a question about c# code optimization and one solution was to use c++ with SSE. Is it possible to do SSE directly from a c# program?
The upcoming Mono 2.2 release will have SIMD support. Miguel de Icaza blogged about the upcoming feature here, and the API is here.
Although there will be a library that will support development under Microsoft's .NET Windows runtime, it will not have the performance benefits that you are looking for unless you run the code under the Mono runtime. Which might be doable depending on your circumstances.
Update: Mono 2.2 is released