join regression failure on cygwin

My Cygwin buildfarm member started failing (hanging, in fact) recently. 
It seems to hang consistently in join.sql and the only way I can get it 
to complete is to kill the backend fairly violently. See 
<http://www.pgbuildfarm.org/cgi-bin/show_log.pl?nm=brown_bat&dt=2009-07-22%2023:10:21> 
It looks like this is the result of the semi-join ordering bug fix Tom 
applied a few days ago, but it is building and running 8.4 quite 
happily, and I think that branch got the same changes, so I'm not quite 
sure about it.

I'm going to try reversing that change locally to see if it fixes the 
problem. Unfortunately, this isn't an easy platform to debug.

cheers

andrew