If they provide good, friendly service, I'll tip shuttle bus drivers a buck or two. I see a number of people tipping them, especially if they bring your bags on/off the bus.
I used to always tip sky caps, because they performed a service. But now, they are just gate clerks who work outside. If it's cold, or if I skip a big line by using them, or the they actually seem to be providing a service, I'll tip. But I no longer consider them to be manditory.