Options on a Mac for a writable, compressed file system?
I ended up going with MacZFS.


Created a sparse-image disk via Disk Utility for about 100GB.  
Formatted it as a ZFS file system by following the instructions in the MacZFS - Getting Started section.  
Turned compression 'on' in ZFS via zfs compression=on [mypoolname]


I tried booting a Windows XP VM off the ZFS file system and it worked. ZFS reports that it achieved a compression ratio of about 1.32x (which means my 10GB VM takes up about 6.6GB on disk). Not sure how well VMWare will play with ZFS's compression under high load, but it seems to working well so far.