well the game has to have "game sharing" equpied but there is a way to make any game multiplayer by sharing the same disk
basicly u start the lvl and pause it then take the disk out, hit yes i want to continue and u share the game
future games will have the nintendo version of sharing games, one copy trasmitted to all the others for limited play
edit:
Oh yeah the psp uses "Universal Media Discs" or UMD sorata like minidiskcs in built in cases that hold 1.8 gb so they are quite nice
, just google a pic of the psp for you people who havn't seen one.. FOR SHAME!