As the buzz is mounting on the Mac OS X platform these days, I decided to buy one and start hacking. On the Windows and Linux platforms, I'm used to compress all my binaries using UPX, thus saving space and bandwidth. Unfortunately, when I tried to download a version of it for Mac OS X, I couldn't find a single link, even on the official web site. So, I decided to compile a version myself.
The task was not straightforward but I was able to build a universal binary Mac OS X 10.4 and 10.5. It should work on 10.3 but I didn't test it. The result can be downloaded from the link below. I hope this will help.

http://www.idrix.fr/Root/MacOSX/upx-3.02-universal-macosx.tar.gz