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F# does not make it automatic, it just makes it easy.
Yet another chance to link to Luca's PDC talk. Eight minutes starting at 52:20 are an awesome demo of F# async workflows. It rocks!
On purity annotations: Code Contracts have a Pure attribute. I remember hearing the some parts of the BCL already use this. Potentially, this attribute could be used by parallellization frameworks as well, but I'm not aware of such work at this point. Also, I' not even sure how well code contacts are usable from within F#, so a lot of unknowns here.
Still, it will be interesting to see how all this stuff comes together.