Naturally, the Xperia Pro is geared toward a slightly different kind of customer than the Xperia Play. Instead of slide-out gaming controls, it gets a slide-out QWERTY keyboard.
Yes, the lower case part of the name is the official name. Sony Ericsson wants to be all hip and cool like that, not unlike a certain iCompany. In addition to the Xperia Play (PSPhone), SE also showed off at least two other entrants to the Xperia family at Mobile World Congress.