So far few details are available. It will be available this summer and It'll be free (Touch Arcade).
It will be a tapping game based on Pokemon music.
What? You expected something more? With Nintendo just announcing the Wii successor and the 3DS coming a long nicely, it is unlikely we will see a full featured title. Obviously, such titles would be very popular with mobile gamers on the iPhone and Android devices.
Short of hell freezing over, simple tapping Pokemon games are all we can expect for now.