There is a post at Apple Discussion forums about using either /etc/launchd.conf or /etc/launchd-user.conf to set the system-wide umask.
Two files were created: /etc/launchd.conf (for system-wide global umask) or /etc/launchd-user.conf (for user-specific global umask). The contents of each were simply the single umask command, umask 002 or (umask u=rwx,g=rwx,o=rx, I can't remember which variant I used - they're functionally the same, though).
There's also a mention of TinkerTool System, which would help do this in a GUI. However, the discussion ends with the caveat that this may work in the Terminal but not in Finder. I have not tested this myself.
