[speeder]

The item that fits under the seat is technically not considered a carry-on piece but rather your "personal item," i.e. purse or backpack. So if they charge for a carry-on bag and you did not pay for one, you might have an issue if you are carrying a small bag. Depends on the gate agent and how awake they are when taking boarding passes...I assume there is some marking on them that tells them what was paid for(?) I really don't know.

Jet Blue hosed me good this summer on Nantucket, they made me check my carry-on at the gate and charged me something like $65 for it. Usually that is free, (when they ask to check your carry-on bag), I could not believe it. It was a small jet and it was leaving, we were basically on the tarmac and I wasn't in a good position to argue but it burned my ass.