If the show runs several more seasons, and it has sufficient popularity, they might.  ER released a soundtrack after its first few seasons because of all its acclaim and high ratings, so I could see TPTB doing the same thing with Castle.  If it's a bug success of a show, you can bet ABC will want to milk as much money out of it as possible.  The challenge is, as southrnbygrace said, to get permission from all the different record companies representing all the different artists.  Tough challenge.