A new Commercial PSP Game thats been released in Japan is Geki Sengoku Musou, heres the info:
Geki Sengoku Musou or Samurai Warriors: State of War is Koei's latest PSP action strategy game. In this version, you can pair with another warrior or soldier to fight the battles, each paired warrior gives you different abilities and power-ups. Koei announced over 200 characters that you can pair with. The PSP version also support head to head battle and co-operative battle with up to 4 players over WiFi.
Sounds awesome, for more info and to get yourself a copy head over to Lik Sang