In a thread about this on squeak-dev back in January, I posted my initial results:
http://lists.squeakfoundation.org/pipermail/squeak-dev/2009-January/133684.h...
After that I got several other combinations to work, but didn't write them down anywhere and would have to repeat the tests. The combination of recent images as host and target is the priority, however.
Craig mentioned in that thread that he runs the simulator in 3.9 using a 3.2 image as the target.
-- Jecel