Well, most “multiplayer games” are going to be on a single unit… I don’t believe ipods or iphones can interface 1:1 with each other.
That said, the iGolf, iBowl are pretty snazzy. They require you to “handle” the ipod in a way that you would swing a club or roll a ball… similar to the Wii Sports lineup.
There are various makes of Chess/Checkers and so on… also good.
I’m still waiting for all of the popular board games to make their appearance. Monopoly on the iPod would be totally boss when waiting in a long line somewhere.