That would actually be a very welcomed twist. However, if it were to happen, don't you agree that they'd probably have to end the show at that point? It's not like Deb would be going out and killing with Dexter (maybe once or twice) and it would take away from a lot of the tension that has been used through out the series.
I do think it's reasonable to assume that the series will end with Deb (and maybe the entire cast) finding out about Dexter's "Dark Passenger". Hopefully they don't go the way of The Sopranos and just fade to black after the reveal without disclosing any of the details of the aftermath.