The simple answer to "What is PlayStation Mobile?" is that it's a way to offer games for mobile devices, through the PlayStation Store, kind of like getting games from the Android Market or the iTunes App Store. But of course there's a little more to it than that.
To start with, the content offered will not be available to any mobile device out there; devices will have to be PlayStation-certified in order to access the service. This makes sense, since it's a simple way to make sure the content will actually run on the device. As of this writing, most PlayStation-certified devices are Sony products, and include many of the Xperia line of smartphones (Xperia Play, Xperia GX, Xperia SX, Xperia arc, Xperia acro, Xperia acro HD, Xperia acro S, Xperia S, and Xperia ion), plus Sony's tablets (Tablet S and Tablet P). The only non-Sony devices are in HTC's One series of smartphones (HTC One X, HTC One S, and HTC One V). Also, while it's not technically PlayStation-certified (because it is a PlayStation), the PS Vita will be able to access PlayStation Mobile.
However, the same press release that announced the PlayStation Mobile program also announced that two more devices would soon be added to the PlayStation-certified line: ASUS TeK Computer Inc's ASUS Transformer Pad, and WikiPad Inc's Wikipad videogaming tablet.
The Where and the When
PlayStation Mobile seems to be Sony's answer to Apple's App Store and Android's Market, providing a way for developers of all sizes to get their games in front of players via the PlayStation Store. The PlayStation Mobile SDK (software development kit) will initially be available to developers in eleven countries: Japan, United States, Canada, United Kingdom, France, Germany, Italy, Spain, Australia, Hong Kong and Taiwan. It will cost $99 USD per year, making it an affordable option even for small developers.
At launch in the fall, PlayStation Mobile will roll out to gamers a few territories at a time, beginning in nine countries: Japan, United States, Canada, United Kingdom, France, Germany, Italy, Spain, and Australia. It will later expand to other areas.
So What Is It Again?
When PlayStation Mobile launches, it is expected to have at least 30 game titles available, both from Sony itself, and from other developers. 56 third-party developers from Japan and Europe have been lined up, plus another 29 from North America, so the service should see regularly updated offerings. Whether or not it can compete with Apple or Android remains to be seen, but if nothing else, it'll give PS Vita owners another whole set of games to choose from. I do wonder, though, how this new venture will affect developer support of the PlayStation Minis line.
Of course, there is a bit of a downside. Down near the bottom of the press release was this note: "In conjunction with the launch of dedicated content for PlayStation Mobile through PlayStation Store, legendary PlayStation games (PS one Classics) for PlayStation Certified devices will no longer be offered." Yes, you read that right, PlayStation-certified devices will not longer have access to the PSOne Classics line. So if you bought an Xperia Play for that reason, you might be a little upset. There's nothing about what will happen to PSOne Classics you've already purchased, though. Presumably, you'd get to keep those, so maybe you can just buy them all now, before they disappear (on the other hand, if they take away the ability of PS-certified devices to even play PSOne Classics, you'd just have wasted a bunch of money). Fortunately, though, PS Vitas will still have access to the PSOne Classics library, so not all is lost (I assume this is because the PS Vita is a PlayStation device, and not merely PlayStation-certified).