As far as I can tell, the hack I was using in rts/Linker.c won't work
on OS X.  Back to the drawing board.

rolling back:

Tue Nov  3 16:05:47 GMT 2009  Simon Marlow <marlowsd@gmail.com>
  * Fix #1185 (RTS part, also needs corresponding change to libraries/base)
  GHC.Conc.ensureIOManagerIsRunning now creates an IO manager thread if
  one does not exist or has died/exited.
  
  Unfortunately this exposed a problem caused by the fact that we have
  two base packages, and hence two IO managers, in GHCi: see NOTE
  [io-manager-ghci] in rts/Linker.c.  The workaround can go away if/when
  we switch to a dynamically linked GHCi.
  

    M ./rts/Linker.c -6 +47
    M ./rts/Schedule.c +4
    M ./rts/package.conf.in +16
    M ./rts/posix/Signals.c -1 +7
    M ./rts/posix/Signals.h +2

Wed Nov  4 10:11:03 GMT 2009  Simon Marlow <marlowsd@gmail.com>
  * hopefully fix validate breakage on OS X and Windows

    M ./rts/Linker.c -1 +1

Wed Nov  4 16:27:40 GMT 2009  Simon Marlow <marlowsd@gmail.com>
  * fix build failure on Windows

    M ./rts/Linker.c -1 +1