I’ve updated all my machines that run Snow Leopard with the update and I haven’t noticed any strange behavior or errors.
If you want to read more on what is affected in the updates I would suggest reading the Apple support site for the (security) update.
There are some reports on changes in the firewall which might affect the workings of your server if you are using the default firewall configuration. I use noobproof to configure my firewalls. It has a better level of control.
Update: the postfix binary is replaced and you need to run a “sudo make install” from the last version you used. This is a reminder on why you keep the compiled stuff lying around on the server and not clean it up after installation.