The VM_Process class that Jikes RVM uses to implement java.lang.Runtime.exec is likely to be the culprit.  I've observed similar behavior on some of the mauve regression tests for serialization that also fork a shell script via this method, but haven't had the bandwidth to investigate very deeply.  If you're interested in doing a little debugging, I think this is probably the place to start.

I think there's a decent chance that this isn't a fundamental or hard bug, just something that hasn't been pushed on that hard in Jikes RVM before.