The site says it's learned that the off-shoot, dubbed Rock Band Unplugged, is currently in development at Backbone Entertainment, and will feature tracks from Blink 182, Boston, AFI, Kansas, Lit, and The Police, plus DLC support, when it arrives this fall.
The gameplay resembles Harmonix's PlayStation 2 games Amplitude and Frequency, with players tapping face buttons in time with an on-screen track. Once players have completed a "phrase" on an instrument, that track will apparently play itself for a bit, allowing gamers to switch to the other instruments and build up their score multiplier.
Update: The Ampltiude and Frequency-esque stylings of Rock Band Unplugged have seemingly been confirmed by Harmonix designer Dan Teasdale, whose Twitter declares that "phrase-based lane-switching beatmatch lives again!"
Sorry to be spelling police, but it's nagging me:
"Rock Band PSP Details: Think Frequency, Ampltiude"...
Thank you :)
I wrote this as fast as I could.