I'm not sure I would have considered Tom Cruise and Angelina Jolie interchangeable, but that's Hollywood for you. According to Variety, Cruise was the latest in a long line of actors who have expressed interest in the project over the years, and after he backed out, Jolie threw her hat in the ring. The deal is pretty much set, and it was thrown together very quickly. I guess when Angelina Jolie says she wants to headline your spy thriller, you bring some people in over the weekend to hammer out the details.
The screenwriter, Kurt Wimmer, will make adjustments in the script to accommodate the protagonist's new gender, and obviously the title will have to change slightly. (Edwin A. Salt = Edwina Salt?) But considering Jolie has already played an action hero successfully in Mr. & Mrs. Smith and this summer's Wanted (pictured), I suspect all that'll really be necessary is a find-and-replace search of the character's name. It's not like Wimmer will need to incoporate a scene where the spy gets her nails done or goes shoe shopping.
So now the question is: After next year's Valkyrie, what will Cruise's next project be? I think Edwin would have been a great fit for him. Of course, f I were the one making Cruise's career decisions, a lot of other things would be different, too, so never mind what I think.