The only times I have ever tipped at the airport were at various skycap outlets across the US (before online check-in was invented), and back in April in one of the JFK DL SkyClub's, because I was trying to unload some singles, and gave the bartenders one dollar each.

Sorry to say, but otherwise, I can't see the need in tipping anyone else at an airport, unless you stop at a wait-service restaurant, where it is always customary to tip the waiter/waitress in the US.

Also...rental car and hotel shuttles should be excluded (IMHO) from tipping, just because its an advertised service, and its not like they run the shuttles every two minutes.