runghc sometimes won't allow redirecting of stdout/stderr

See attached test.hs. Here's stdout working normally:

$ runhaskell test.hs | cat
output
$

But after uncommenting either the cmdArgs line or the defaultMainWithArgs line, stdout can't be captured:

$ runhaskell test.hs | cat
$ 

It does appear on the console if you don't redirect though. The same goes for stderr. This prevents me testing uncompiled scripts with shelltestrunner. I'm on mac os 10.5.