I haven't been watching this, but I have one question: does prefetching actually *work*?  Do you have benchmarks (or better still, actual library/application code) that show some improvement?  I admit to being slightly sceptical - when I've tried using prefetching in the GC it has always been a struggle to get something that shows an improvement, and even when I get things tuned on one machine it typically makes things slower on a different processor.  And that's in the GC, doing it at the Haskell level should be even harder.