Had this happen once (though not on a one-way ticket) leaving Melbourne. I was angry that I was not able to check in online and was sure that I was going to end up deep into the upgrade list (there was one seat left). Much to my surprise, when I checked in at the airport at about T-2 hours, I was at the top of the list, which was about 20 people deep -- ended up getting the seat... I've really stopped trying to guess where I'll end up.