I’ve just finished updating my test and production server with Mac OSX 10.5.8. The update went without any problem and my setup was not affected in a negative way. Even postfix kept working without a problem.

If you want to read more on what is affected in the updates I would suggest reading the Apple support site for the 10.5.8 update and the Security Update 2009-003.

In case of doubt, please make a full bootable disk image backup with Carbon Copy Cloner before you start. There are some people having reported problems with previous update so better be safe then sorry!

Good luck