RE: When can a child fly without a car seat!?!
We just recently flew cross country and let DD fly without the carseat (gate checked it). She just turned 3. Honestly, it was a bit of a struggle keeping her in her seat at times, but she did mostly OK. We did this as "practice", since we are flying to Europe in a few weeks and are not bringing a carseat at all (we are only using public transport in Europe). She actually kept asking for her seat, since she is used to sitting up much higher, but she seemed to get used to it by the end of the flight.
One thing to think about is that if your DD needs the carseat to sleep. My DD generally does not sleep on airplanes anymore. I mourn those days, but its true. If she slept on the plane, I would probably still lug the seat, but since she doesn't, I have lost a big motivator to take the seat on the plane.
HTH,
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)