Ben
perhaps it is too late to suggest things for GSOC --
but stephen tetley on a different thread pointed at aaron turon's work, which there's a very interesting new concurrency framework he calls "reagents" which seems to give the best of all worlds : it is declarative and compositional like STM, but gives performance akin to hand-coded lock-free data structures. he seems to have straddled the duality of isolation vs message-passing nicely, and can subsume things like actors and the join calculus.
http://www.ccs.neu.edu/home/turon/reagents.pdf
he has a BSD licensed library in scala at
https://github.com/aturon/ChemistrySet
if someone doesn't want to pick this up for GSOC i might have a hand at implementing it myself.
Keep use in the loop if you do. I have a very nice application that has been needing a nicer approach to concurrency than IORefs but really can't afford STM. Cheers, - Ben