Thanks everyone for responding!
RGG: Are those clear airline bags free? Do they seal up pretty good and everything?
Adventuremom: So I can bring a big ol' bag of car seats through security?
I would prefer to gatecheck them. I didn't know you could do that. There will be 2 adults, a 2 year old and an infant (in lap). I don't think I want to give up the space for a carseat, plus the 2 year old has been on many planes and likes to buckle up and sit like we do. I don't think a car seat would be that helpful.
I am tempted by the airline bag, but the maclaren double stroller bag is tempting. We also have a maclaren double stroller so I think it would be a good purchase.
I thought to damage a car seat it would need to be more than just dropped a little. I would think gatechecking it or even checking it would be ok. But I have only flown a few times with a kid, so I don't know too much.
Thanks for all your good advice guys. I love this forum it has helped me so much!
Most airplane emergencies are runway emergencies, which have physics similar to a car crash. Would you put your two year old in a lap belt in a car? I'd be surprised if the lap belt, even if it's snug on him, hits on his hips, not on the soft part of his abdomen with his soft organs right behind there. Not to mention the huge buckle sitting on his belly button for his body to ram into. If there's time you'll be asked to wrap your lap baby in blankets and put them in the space under the seat in front of you. Compartmentalization and padding. Otherwise you'll crush your baby (have you seen the video of an unrestrained mother and her baby on her lap?). Say you weigh 130 pounds. Say the runway incident is happening at landing, 150 mph. That would make you all of a sudden 19,500 pounds. Your upper body of course has most of your weight, with your organs and everything, and that'll be jackknifing over the lap belt. You don't want your baby right there. I would HIGHLY recommend you bring both seats (the FAA even recommends seats, and every flight attendant I know thinks seats are a good idea, even if they don't know what the manual says about them). If there's an empty seat on the plane you'll be allowed to install the infant seat free of charge. If not you gate check it.
Have you ever seen the Mythbusters where they drop the team from five feet to test the brace position? The brace position will help, they say that, but from five feet their arms are up over their head, their heads aren't on the seatback in front of them anymore, and they slam down again. No way you're holding onto a baby if the fall is from more than five feet. Not to mention the coming down again part. I think it'd be unlikely baby would land on your lap again. Not to mention they were all sore from a five foot drop, they had that much force on their bodies, and they weren't trying to hold something.
I'm confused by what you said, you don't want to give up the space for a carseat? What space? The seat would go in the seat you bought. If it's RFing it's quite comfortable, you get a nice armrest.

If it's FFing, well, then you can lean on it to sleep. And if you have an infant seat and a toddler in a seat, and two adults, they're going to break you up probably. Infant and one adult in one row (carseats MUST go by the window and they CANNOT go in an exit row or the row in front of an exit row) and toddler and other adult in another. Or toddler by window, infant in middle, adult aisle and then other adult somewhere else. That's with two child seats. I'm very confused by what you say "give up the space." If you need a narrow carseat for flying, if you do it often, then I would recommend the Radian. It's narrower than a plane's seat, low enough to allow the tray to be used, folds up and can go in a backpack for through the airport, and has an open beltpath that is easy to install and uninstall the big airline buckle through. You'll be able to use the armrest from the plane, it'll be just like sitting next to another person, instead of a bulky carseat. And if for some reason you don't use it, you can put it in the overhead bin, where you don't need to worry about damage.
I HIGHLY recommend you take the seats on board. Even gate checked the seats are in the luggage compartment. And if your suitcases above can shift during flight, think about what's going on down below. Damage can occur, even gate checking, not only from the guys throwing them onto the conveyor belt. I absolutely cringe when I see carseats coming through on the baggage carousel. And I nearly want to shake parents who bring their kids on board without a seat. I know most don't know better, but I just want them all to know. We did it to, before we knew better. Lap baby, checked the seat, rented carseats at the destination. Then I learned and I will NEVER do it again. Piper's too old to travel by lap anymore, but if I had another baby and for whatever reason couldn't get a seat, either cost or the plane is full or something, I simply wouldn't go. Or I'd drive if possible.
Not to mention, it's far more comfortable for kids to be in their carseats. Those airline seats are not made for them. I can't imagine flying more than an hour with a kid in a regular seat, the seat is so big and they're so tiny.
It's great that he likes to buckle in just like the rest of you. Would you let him just wear the seatbelt in the car just because he likes it? It's not made for him, it's not safe for him. Have him buckle himself in in other seats on the plane while you install his carseat. Then let him buckle into his seat in his harness, just like the pilots do. No offense, but if the pilots do something I bet that's cooler than what you do. hehe See if you can get him into the cockpit to see the pilots' harnesses. Or show him the flight attendants sit backwards with a harness. Seriously, adults are so unsafe on a plane it's not funny. And that's with seats and belts made for us. A two year old simply is NOT safe. Just because you're not getting yelled at doesn't mean it's ok.
Wendy