There are five titles currently available, all priced at £3.99: Syphon Filter, MediEvil, Cool Boarders 2, Destruction Derby, and Jumping Flash. They don't have an American price yet, but that will surely change once the Xperia Play launches in the States.
Two important questions remain unanswered: How big are these games? The Market descriptions say the games are only 5MB, but that sounds incredibly unlikely -- and more importantly, will it be possible to 'spoof' the Xperia Play and download PSX games onto other Gingerbread-powered phones, like the Nexus S?
In other news, the PlayStation emulator PSX4droid was recently removed from the Android Market. This obviously has nothing to do with today's release of first-party PlayStation games.