I really need to go look through my bookmarks, but I've recently been following a very good fic where Dean is Lucifer in the same way that Anna was human. Will see if I can find it, if you're interested.

Edit: ah well, checked my subscriptions and found it easier than I thought.

Sympathy for the Devil (and Dean Winchester) by TardisIsTheOnlyWayToTravel

Summary: After the final Seal is broken, Dean discovers that he's actually Lucifer. He's not really sure how he feels about that. The forces of Heaven and Hell aren't exactly making things easier.