Originally Posted by
raucous
Yes, your itinerary does cancel, but frankly so what? In my experience there is little consequence other than when you arrive, you must go to an agent's desk and they will be able to recreate the itinerary and place you on standby for the next flight. Calling ahead of time doesn't help you get any further ahead on the standby list as you only go on standby at check-in at the airport. And the flights are full you are not going to make it on a later flight regardless of whether you called in ahead of time or not.
I don't remember how long the flat tire rule is (I think either two or four hours), but if you arrive sufficiently after your first flight and didn't call in advance, you'll find that your ticket has not only been cancelled but that it no longer has any value and you won't be able to standby. Maybe the AAgents will be nice and ignore this, but they're not obliged to.
So, in most cases just showing up later is probably fine. If the gap between flights is considerable, though (e.g., morning to evening), you'd be smart to call.