generative modeling for Rhino
Is multi-threading even possible in Grasshopper?
Not entirely true. Most of the geometry operations are threadsafe. Just don't start messing with the RhinoDoc while in threads.
I've been working on multithreaded python scripts lately and may have something to show in the near future.
also you may want to check out the Parallel library in .net 3.5 or 4.0. they allow you to do simple for and foreach loops without the need for thread managment.
the part i find difficult is making everything synchronous because current easy methods for multi threading are not. meaning that that the result will be out of order which for a lot of things it is important to maintain order.
Here's a python sample that uses the Parallel library that Robert mentioned.
You're going to want to have at least the May 31st, 2011 WIP of Rhino 5 to test.