(copied from another forum)
It would be perfectly fair to put up a pay station in the gate area. You would pay for checkable bags whether or not you succeeded in carrying them into the cabin. You would not pay for qualifying carry-on bags whether or not some got gate checked. If you skipped the pay station and headed for the jetway, you might get singled out and sent back to pay for items. If time got short, the pay station might close up without processing everybody who then proceeds to the jetway.

Actually they should have started charging for carry-ons before this first checked bag issue came up. Speed up boarding and/or enjoy a big cash cow.

Travel tips: http://members.aol.com/ajaynejr/travel.htm