I'm still waiting for my Direct2Drive preorder code as well, I'll start a new topic when it becomes available if nobody else does. My room mate preordered through steam so I'll ask him when he gets that as well.
Edit: You know what, on second thought I bet steam just recognizes that brink was preordered, and the codes given from Direct2Drive are also recognized as "preorder codes".
It's likely us D2D users *will* have to input a code. Steam users will have that code already part of their install, and it will load as a pop-up saying it's applying it when the game is first launched.
I've never seen a D2D game incorporate the pre-order bonus into the retail game key. That would require them having special "pre-order" codes. Which, afaik, they do not.