What you have written seems to suggest that you have only applied the umask fix to your machine (the server), and not those which your users are working from.

Although in your case you would also need to apply the fix to your "server" since you use it as your workstation, it is important to realise that the primary umask setting for a file is determined by the computer that saves it. The fix that Apple have implemented in Mac OS X Server 10.2.6 and above involves the server "overriding" this initial umask, essentially by creating a new file with the override privileges instead of those set by the individual workstation. In your more adhoc setup it is only really possible to solve the problem at source i.e. your users' computers.

That said, it might also be useful to check the SharePoint Preferences have been set to allow files in the shared directories to inherit their permissions from their parent directories, and maybe doing a quick sweep of those shared directories with BatChmod to correct any errant permissions.