All the discussion above about technical considerations and the GE exclusivity on the larger 777 models is a bit wide of the mark. It was a commercial negotiation driven by Boeing that led to this, based primarily on what would offer the best financial return to Boeing. Which is a fair enough way to go (well, as a sales person I would say that).
The 777-300ER has hit good sales targets for Boeing, but the 777-200LR has been a poor performer. It's a limited market for ultra long haul, as Airbus also demonstrate with the A340-500. Rolls Royce got the exclusivity on that by the way, it cuts both ways. It is not in the airframe manufacturers interest to have a limited-production model dissipate technical resources over two or three engine models, if they can avoid it.
It is, as always, an interesting spectator sport to see two supremely professional organisations slug it out commercially. For those who have been around longer the most surprising thing is the demise of Pratt & Whitney from former supreme leader to one who is steadily vanishing. It must be years since P&W actually made a New Business sale, all they are left with is a half share in the GP7200.
In the City AM financial world daily newspaper there was an informed story yesterday about financial difficulties at GE that are causing questions, especially at their leasing arm GE Capital, who have financed a good proportion of the world airliner fleet, so more to watch there.