FYI for the op and all those traveling with car seats...
The FAA issued an informational memo to all US carriers in February of this year. Here it is in PDF format:
http://www.faa.gov/other_visit/avia...afety/info/all_infos/media/2009/info09002.pdf
I strongly advice anyone planning to bring a car seat onboard to print this out and put it in your carry-on.
"No operator may prohibit a child (an individual who has not reached his or her 18th birthday) from using an approved CRS when the parent or guardian purchases a seat for the child, the child is accompanied by a parent or guardian and the child is within the weight limits for the CRS."
If you paid for a seat on the plane, and have a car seat, and your child falls within the weight limits they MUST allow you to use it.
"If an approved CRS, for which a ticket has been purchased, does not fit in a particular seat on the aircraft, it is the responsibility of the aircraft operator to accommodate the CRS in another seat. The regulations also permit an operator to use its discretion in identifying the most appropriate forward-facing passenger seat location, considering safe operating practices."
The second part of that paragraph is important. They must accomodate your car seat, they do NOT have to give you a specific seat (like a bulkhead seat for example) that you want, IF they have another location that your car seat also fits in (like the last row for example).
And also, in their examples:
"An aft (REAR) -facing CRS that cannot be installed properly because of minimal pitch (distance between seats) between rows can be moved to a bulkhead seat or a seat in a row with additional pitch."
So if you have a 30lb child in a Radian, for example, and you want it rf'ing (allowed per the first paragraph above, most Radians have a stickered 35lb rf'ing limit, brand new ones will be stickered with a 40lbs rf'ing weight limit) then they must move you to a seat in the plane that will accommodate the seat.
And for those having difficulties with installing seats ff'ing and the buckle interfering, ask for a seat belt extender. That will help.