OSX server 10.6 and umask
umask is a user- and process-specific setting and can be overridden. Setting a umask on the server does not affect any clients that accessing the shared folder - each of them will use their own default file creation permissions.

If you want to set default permissions on a shared folder, the answer is to use Access Control Lists (ACLs), which are much more flexible.


Here's a somewhat terse support article from Apple on setting up ACLs in Snow Leopard Server.
And here's a slightly friendlier guide.