Percival has released a multiplayer version of GTA Pong, heres some detail:
"This one is good if you have a friend nearby. It's basically GTA Pong without the 'computer' opponent. You hold the PSP vertically, with one end of the PSP in your hand, and the other end in your friend's hand. Move the paddle left and right and walla! Multiplayer using one PSP. Innovative!"