I’ve just finished updating all my servers with Security update 2009-004. Experience from the last updates made me check the postfix config for changes and no it looks like they are leaving the postfix configuration alone. Maybe because this update patches a single vulnerability affecting the BIND DNS server only.
If you want to read more on what is affected in the updates I would suggest reading the Apple support site for the 2009-004 security update.
In case of doubt, please make a full bootable disk image backup with Cabon Copy Cloner before you start.