Just got back from Orlando with our 4 year old DD and 7 year old DS. DD isn't quite big enough for a booster, so we bought a new car seat for her a month or so ago that goes up to 50 lbs - and I did all my research and she actually flew in her carseat. I was VERY leery about checking her carseat, as I've read and heard horror stories about how they are treated. AirTran initially gave me a bit of an issue, but I firmly stated that her seat was approved for use on airplanes, and being a Cdn seat wouldn't have the US sticker they were looking for... so they agreed to let her fly in it. For DS, we bought him a backless booster around the same time, and we packed it in our luggage.
It was great because as soon as we got our rental car, we were ready to go!
Darlene