Charlie Allom and I have collaborated on a script that does 
cross-platform builds of NetBSD.  It can be configured to do multiple 
platforms but currently it handles mac68k, macppc, sparc, and i386.  
The output is an iso image that can be burned to a CD that is bootable 
on i386, sparc and macppc.  On NewWorld Macs the "C" snag key even 
works for booting the CD.  The script can be run in a cron job and can 
be configured to do a CVS update as part of the build.

The script is still Work-in-Progress so there may be some glitches, 
although it has been working for us.  (I'm using it on a 1.5.2 system 
to build -current snapshots.)  If anyone is interested in trying it out 
it can be found at:

     ftp://murphy.dyndns.org/pub/cross_build

Comments and suggestions for improvement are welcome.

Enjoy,
-bob