Found this over at the Apple discussion forums...
By default, when saving a file or creating a folder, the object will be saved with permissions of 644 or 755; or a umask of 022. However, I need to have a umask of 002 such that my files turn out as 664 and 775. Sure the .cshrc file will sort that out for me in the terminal but not for applications.
The solution is to place the umask in your /etc/rc.common file, like so:
##Not sure yet if there are any things to watch with this (like your application must be Cocoa, etc.) but seems to work for me.
# Be strict
##
#set -e
set -u
umask 002
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002111405523151