Interesting topic.

While I would never think of sneaking into first or business when I was seated in coach, this discussion raises more questions ... what if the check-in agent forgets to collect your upgrade certificate and you don't mention it, is that stealing? Is it different than if a clerk at a store forgets to take your money when you buy say a TV?

What about if you book a reservation claiming to have a confirmed upgrade cert but then you "pay" with a less-valuable unconfirmed cert? Isn't that stealing, if not from the airline then from a passenger who might have been entitled to the upgrade ahead of you? I found it interesting (OK, disturbing) that the last time this was mentioned on a thread the outrage wasn't about doing it, but about revealing people's tricks.

Few things in life are clear cut it would seem.