Default profile with Active Directory As a warning: I'm no specialist at all. I work as a student here and trying to help to fix the Mac environment. I might use the wrong terminology at times but I try to be as clear as possible. 

Assume we have a bunch of users with windows network accounts that they only ever used in our windows domain (we use AD). Now they start using Mac (joined to the domain) and we wanted to have a default profile that every user gets the first time he logs in with his network account with a preset background and lets say a shortcut on the desktop.

What would we have to do that a user that logs in on a mac the first time that a copy of our default profile is used and then stored in his personal network folder.
Have you looked into creating a local user template? Here for example: http://gigaom.com/apple/how-to-create-a-custom-user-template-in-snow-leopard/

The template folder is copied to the new user folder at first login. The problem now is how to deploy that user template to every new mac, but that's easily solved by other means (you can create a package and distribute it via ARD/munki/Casper, you can use puppet - which you should do anyway).

Mind you that every change to the User template will only affect new users; existing users have to be managed by other ways too (MCX via augmented AD schema, for example, is what we use).