Check out Reactive Extensions to .NET (Rx). Looks like it includes “a back ported (and unsupported) release of Parallel Extensions for the .NET Framework 3.5 in the form of System.Threading.dll”. So, if you don’t have Visual Studio 2010 beta handy you might check it out and let us know how it goes. While you are there make sure you check out Rx as well as it looks an interesting and useful library once you grasp its concepts.
See the related blog post.