Setting NSUmask in Leopard is done differently than in previous OS X releases (older hints on NSUmask). In 10.5, NSUmask is gone. To set a default umask (for both shell and GUI apps), edit /etc/launchd.conf and add this line:
umask 077
where 077 is the new default umask. If nothing is there, the default is 022. Note, the /etc/launchd.conf file umask "trick" should work in Tiger too, but I didn't test it.
Note that 10.4 uses "User Private Groups" (see this document for more), but still sets the umask at 022 (this results in new files/folders having a permission of 055 rwxr-xr-x). In Leopard, the default umask is still set to 022, but no longer uses User Private Groups. This means that all users are a member of the Staff group (GID 20), and files are set to r-- for Staff by default. If you want to lock down the files on the Mac so only the creator can access the files, you need to set the default umask to a stricter number (077 for example). In Tiger, if you set the umask to 027 (NSUmask 023) you could not change the time zone using System Preferences.

