queenie - TSA has a device that reads the barcode on the phone, shows them the name on the BP and other info (flt # and date, maybe?). They check your ID but obviously have nothing on which to write. When this was first available the ID checker had to verbally tell the screening line agent about the ID having been checked, in one or two places the TSA ID check agent gave me a piece of paper to show at the screening line, now it's just waltz on through after ID check.

I've wondered if you check in online, then miss the flight, can you get relief, or do they consider you as having used your ticket? I don't recall seeing any unsignin option on the sites.

In the incident I described above, I had checked in online the day before and then missed my flight. I arrived at the gate just as they were closing the doors. I was booked on the next available flight--no extra charge. This was on US Airways, and I've read they have a 2-hour "rule", i.e., if you arrive at the airport within 2 hours of your scheduled flight they will put you on the next available flight at no charge.

Policies differ, though. Some airlines charge you a change fee for missed flights, plus the difference in cost for a same-day walk-up ticket. This could end up being several hundred dollars, especially if there is more than one passenger. Other airlines charge a flat fee.

If you are talking about checking in online and then not taking the flight at all, i.e., canceling it, then you will probably be charged a cancellation fee that can be as much as $150 per ticket. The remaining balance usually can be applied toward another ticket within a year.

Each airline is different, though, so if you are running late or need to cancel the flight altogether, I would call the airline ASAP.