thread blocked indefinitely

I'm getting intermittent "thread blocked indefinitely" errors with GHC
5.04.2.  My program uses modules Concurrent and Posix heavily.  In
particular, I fork processes (now with GHC.Conc.forkProcess) and handle
SIGCHLD signals to determine when the processes have finished.  The
symptoms seem similar to "evacuated object entered" problems I had with
GHC 5.02 and an earlier version of my program.  Any bugs or gotchas I
should know about?  Suggestions about how to collect information with
which to debug this problem?  (I've heard about +RTS -D1, IIRC, but have
not tried it.  Is a debugging RTS available?  Is it possible for a mere
mortal to build one?)

Dean