We have a Marathon and a Roundabout and flew with the Roundabout since it's smaller and easier to transport. My daughter was 9/10 months on our trips so we put it rearfacing and it fit without a problem (on Continental, Airtran, and Southwest). On one Airtran flight, the flight attendant told us the seat had to be forward facing so my husband and I said that she's under a year and only 18 pounds so she's supposed be backwards and she raised her eyes and huffed off, sent another flight attendant back who asked us if our seat was belted in and asked to see it and then walked away. I guess they thought we just rigged it in there backwards, who knows, but ultimately we left it that way and it was fine. The 2nd Airtran flight (where I was alone with my daughter because my husband and mom mistakenly were booked on a different connecting flight) the flight attendant insisted I show her the FAA sticker at the door to the plane (while carrying my infant, diaper bag, and wearing the carseat on my back in a carrying bag) or she would not let me board with it. She said she could not come to my seat to check it (though I was in the 2nd row) and I had to show her there. Talk about a hassle. And our bag is one that has to be like totally unzipped to get the seat out, so all the toys and tissues fell out all over the plane and everyone was held up. But anyway, my point is that the sticker is on the side of the seat, the right side I think. It's in red lettering on a sticker with lots of black writing about the safety of the seat, etc.
We flew with her infant seat up until Christmas since it's much lighter and easier to carry, but she hated that seat and is much happier in the Britax seats, so even though it's harder will used it. Of course soon we won't have another option as she'll be over the weight limit for her infant seat anyway (she's been over the height limit for a few months, but the car seat tech said it was fine for airplane use since she was under the weight limit).