Here's the official word from Disney in
Disney's Magical Express FAQs - Planners:
Q: Are Bell Service gratuities included for the luggage delivery to the Resort room?
A: Yes, but for the inbound service only. Gratuities are not included if the Guest elects to use Bell Services for luggage assistance when departing.
There's no word about tipping the motor coach drivers. I wouldn't be surprised if there's a cup in the front of the coach -- and that most passengers will ignore the cup. If the driver is not giving you personal attention, such as helping you with your luggage, it's really hard to make a case that the driver should be tipped.
There's also no word about tipping the Resort Airline Check-in employees (who actually work for BAGS, Inc.) who will check your bags at your resort if you're flying home on one of the participating airlines. I wonder if they're tipped like airport skycaps (who depend on tips) or like airline ticket counter employees (who do not receive tips)?