Any game that is 'Multi-Platform' runs on dedicated servers, exclusives are run 'Peer-to-Peer' (on PS3), they only use servers as a link to each PS3 (in GT5s case).
Wait, what?
Are you talking about cross-platform online gaming (as in different systems playing online together (such as final fantasy mmo), or multi platform games such as CoD (which uses peer 2 peer, one person in the lobby acts as host).
Multi platform games use both dedicated play servers ( battlefield 3), and peer 2 peer (CoD), gt5 uses two forms of peer 2 peer, fixed host means only one person in that lobby is host, non fixed host means everyone in that lobby is a host (but only the Creator has admin rights).
Exclusives use both as well, mag uses dedicated play servers, and I already explained gt5.
Heck even cross-platform games can use both, 4x4 evolution was cross-platform online, PC, Mac, and dreamcast could play online with each other and it used peer 2 peer.
The only time you touch a dedicated server in a peer 2 peer game is when you first connect online and looking for people to play against, one is a matchmaking server like in CoD, and the other is a lobby list server such as in gt5, in gt5 the lobby Creator also pings the lobby list server so that the lobby can be seen and found by others regardless of what host mode you have set, so if in fixed host mode when lobby host leaves the lobby ends, in non fixed host the Creator still pings the list server, when he quits all the functions pass to the next in line. If for any reason the admin of the room lost connection to the list server the lobby will end.
You have dedicated play servers (which is what you play on when you join a online game) and matchmaking/lobby list servers for peer 2 peer (think of it as a IP collection server so you can find people playing the same game). With dedicated play servers you still have to connect to a master list server to be able to see and connect to a game server (which is the same thing as what gt5 uses to list the available public lobbies).