There are no car seats allowed on any bus that I know of. That includes school buses, yet we still send our children on those; right? I was just making the point that children are on buses many of an occasion without a carseat. ME is a big coach bus. I still bring carseats on the plane to keep my kinds a bit more secure. My DS2 is a houdini with seatbelts but hasn't mastered his carseat harness yet. I will stowe it under the bus for the ME and then in the closet at the hotel. That way, IF we end up renting a car for a day I have the carseat just in case.
Actually megabus has seatbelts and they reccommend that any child over the age of 2 and under the age of 7 be in a car seat. Oddly enough under the age of two they want them on your lap unless there is an extra seat available.

, when we brought our 2 year old on the Magical Express, she kept saying over and over, "no car seat?" It was hilarious, I think she said the whole hour we were on the bus, and then again the next day on the Disney transportation bus. After a couple days she stopped asking, but when we got picked up by my mom and she asked her about the trip, one of the first things she said was "no car seats!" 