Unless "Terminator: Salvation" just bombs terribly (and from what I've heard the film is passable; not great but not a disaster), it's a franchise that's going places and MGM wants to be wherever that next destination may be.  Due to a convoluted series of deals and court settlements, the studio gets first-look at the fifth installment of the franchise even though Warner Bros. is handling the domestic distribution of "Salvation" with Sony controlling the overseas distribution. 

The Variety article* explains that while a fifth "Terminator" movie would go well with MGM's line-up of Guillermo del Toro's "Hobbit" movies, Darren Aronofsky's "Robocop" remake, and David Cronenberg's "The Matarese Circle", it's going to carry a significant price-tag since WB had to pitch in $50 million for the domestic rights and Sony had to put in $75 million for the international. 

Basically, the question mark on this article is a formality.  While MGM hasn't officially signed on to distribute "Terminator 5", they'd be nuts to turn it away and the bigger question is just trying to find out how much money "Terminator Salvation" can make when it has to compete with "Star Trek", "Night at the Museum 2", and in its second weekend combat the behemoth that will be Pixar's "Up".