iOS 3g Network Connection for Multiplayer Game

I know on wifi you can use bonjour to discover other iphones for a multiplayer game but what is the correct method to connect over 3g? Does there have to be a central server or can you just give the other player your IP address on the 3g network to make the connection?

Basically is there any firewall on the 3g networks that prevent incoming connections to host a game?

Thanks in advance.