Details about gameplay have yet to be released, although we don't expect to see a full suite of tiny plastic instruments to clip on to the PSP. What is known is that Unplugged will use the PSP's Wi-Fi connection to access an in-game music store.
Update: Primotech has some possible gameplay details. According to the post, Unplugged will use the left, up, triangle, and circle buttons to control instruments, switching from instrument to instrument with the shoulder buttons, and playing "phrases" of each instrument to maintain a score multiplier. This sounds exactly like Harmonix's Frequency!