I also would like to know what the rules are supposed to be. I had a bad experience back in 2000, actually with UX (presumably ACA) at IAD.

I was supposed to fly out for the weekend for a birthday party. I showed up a little under an hour before my late Friday flight and proceeded to the gate to check in. The agent says to wait a few minutes because the flight might be canceled due to weather. After about fifteen minutes I return to the counter and ask what the status is. He then says the flight is oversold and he cannot check me in; he gives me a departure management card. Departure time comes and goes; within a half hour word comes down that it is officially canceled due to weather.

All United flights are booked solid through Monday (!), so they try to accommodate me on other airlines, which are of course full as well. Since the first available flight leaves Sunday around 7pm-- when I should be boarding my return (talk about trips in vain)-- I ask for a refund. The agent says I can request one on the phone and to step aside so he can assist the numerus people in line behind me.

I go home and call United and guess what? The rep says I never checked in for the flight, and it was a nonrefundable fare, so I am not entitled to a refund. I say I have a departure management card that says I was at the airport and tried to check in, well before the flight was closed. He says it confers no legal rights to me, it is not a boarding pass, I am a no-show. Well, I about blew my stack. I calmed down enough to call back about twenty minutes later and got a different rep who did issue me a refund, which was only a couple hundred bucks anyway-- but considering this was an out of pocket leisure trip, the experience was enough for me not to fly United for two years and ensure none of my friends did either.