Once when I was moved on to a heavily booked KL flight at the last minute (shortly before check-in closed) I was given a boarding card (at the check-in counter) without a seat number on it. When I got to the gate (and presumably they'd closed check-in and made any necessary seating changes) I was given a new boarding card with a real seat number. Unfortunately, I had to return the first boarding card, so I don't have a record of what it said, but I think it must have been either 'Standby' or 'SBY'.

This thread in the Quantas Forum suggests that SBY is a standard abbreviation for 'Standby' : Assigned seating on LA code share

Hopefully this means you're in with a chance of an op-up...