I’ve just finished updating my test server with Mac OSX 10.5.7. 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 10.5.7 2009-02 security update.
In case of doubt, please make a full bootable disk image backup with Cabon Copy Cloner before you start. There are some people having problems with the update so better be safe then sorry!
I will update my production machines later the coming weekend when I’ve got some more spare time.
PS. I just updated my production server and everything works like a charm. No problem what so ever.