For the most part, the use of iPhones in TV and Movies are related to the actors. Unless the production company is getting product placement money for a particular phone (like James Bond using Sony Ericcson Phones, because the series is owned by Sony), then often the actors use their personal phones, one from the prop closet, or one from another staff member. I know that Sean Murray's use of an iPhone in NCIS results from the fact that he stood in line in LA and bought one when the came out. It also happens that it also fits his character, so the producers keep featuring it, often doing impossible things they mimic with Quicktime movies.
Just remember, not all the products shown on TV or in movies are paid endorsements.
TEG