Sony's folks have said several times that they're interested in supporting more formats on the PRS line.
Mobipocket carries the obvious "Amazon doesn't want to play with others" fetter, but why wouldn't eReader/Fictionwise want to do so? Especially considering this new transfer of ownership. They're in the business of selling books, not hardware, and customers won't buy those books if they can't read them on their devices.
Besides, making eReader run on the PRS would likely get them a version that would be very close to running on the iLiad, Cybook, etc., and vice versa.
