unsafePerformIO and cooperative concurrency

Given that unsafePerformIO is (perhaps) to be part of the Haskell' standard, the combination of unsafePerformIO and cooperative concurrency introduces some interesting cases that will probably have to be declared to be undefined in the standard - what happens when you yield or do some I/O inside unsafePerformIO? I guess a concurrent foreign call executed from inside unsafePerformIO doesn't work either? What about a reentrant one? (I know that reentrant foreign calls currently don't work inside unsafePerformIO in Hugs, IIRC it can lead to a segfault). So does this mean that a foreign call that doesn't return an IO type cannot be declared concurrent? Cheers, Simon