OS X Server & Time Machine for Active Directory Users
Ok - I feel foolish.  I found the answer within minutes of posting the question.  Here is the answer:

https://discussions.apple.com/message/24844922#24844922

I had to enable 'File Sharing' on OS X Server as well, add an explicit folder (or drive) where the backups would go and add explicit permissions to that folder. You can allow 'everyone' to access it, but in my case, I have an AD Security group that contains all Mac users in the org.