(Legacy answer – it seems the backport for 3.5 is no longer readily available.)
You can’t use the full Parallel Extensions, no…
… but if you install Reactive Extensions for .NET 3.5, that comes with a version of Parallel Extensions, so you can use that. I don’t know how much of PFX is supported, but I suspect there’s enough for most people. (There are some details in the blog post, but that was from 2009… I don’t know about any changes in 2010 which may or may not have been backported.)
Note that this is unsupported, too – probably fine for hobby projects, but if I wanted to use PFX commercially, I’d upgrade to .NET 4.