Here's the low down: your seat has to say it is approved by the FAA (it should be somewhere on the seat on a sticker, like where they have the warnings and stuff). As long as it does, you are good. Most of the time they don't check but I did have a flight attendant check it once so it is a good idea to make sure. Even if you can't have the kids sit in them on the plane, you can always check a carseat and it doesn't count as your 2 allowable pieces of luggage. Also, you can check it at the gate, just like a stroller, if you don't feel comfortable with the way they handle checked luggage. I have flown with carseats A LOT. There is no reason to rent one. :D HTH!
Julie in the Springs