Today Sony announced, among many other titles, an exclusive "Rock Band" title for the PSP called "Rock Band Unplugged." From the press release:
Publisher support for the PSP system will continue throughout the year with Rock Band Unplugged, launching exclusively on PSP. MTV Games and Harmonix announced separately that gamers will be able to take Rock Band Unplugged on the road with the first ever portable version of the popular music and gaming experience. Rock Band Unplugged will take center stage as it leverages the PSP system's Wi-Fi capabilities and becomes the first PSP game to include a fully-featured in-game store for downloadable content in the North American and Europeans markets.
No other details were provided, and MTV Games declined to comment further at this time.