Navagon: The cops know his gamertag. They don't know where he is, but they know what he's doing
(playing MW2).
Wishbone: Now, if only MW2 had had actual server support, instead of the crappy random-client-is-server gimmick, that might actually have enabled them to track him down.
No real difference, unless you mean BC2-style servers where they are all at the same few farms. You would still need to request information from the server admin. Only difference is it is a user who would send the log, rather than a facility.