See: Mac OS X Server v10.5, 10.6: Setting a custom umask
This is how I like the Mac setup because the MBP I use has two accounts, both of admin level. W/o this change, the other user can't do anything to files you've created, even if they are shared with you. Incase that article ever goes away, here's the important part of it:
Umask for user applications
In Mac OS X v10.5.3 and later, you can create the file /etc/launchd-user.conf with the contents "umask nnn". Do not include the quotation marks and replace nnn with the desired umask value, such as 027 or 002.
This will set the user's umask for all applications they launch, such as Finder, TextEdit, or Final Cut Pro, and control the permissions set on new files created by any of these applications.
Umask for system processes
In Mac OS X v10.4 and later, create the file /etc/launchd.conf with the contents "umask nnn". Do not include the quotation marks and replace nnn with the desired umask value, such as 027 or 002.
This will set the umask for all processes. Changing this value is strongly discouraged because it changes the permissions on files used by the system software. If the permissions are too restrictive, dependent software may not work. If the permissions are too open, they may introduce security issues.