I think the final nail in the coffin may be proposed landing fees from Heathrow and perhaps others. Due to increased wake turb separation requirements and the huge capital work program to accomodate it, the landing fee for an A380 will be around double that of a B747 (helped by the severe lack of airports capable of taking the aircraft). I understand a 5 tonne blow out in empty weight makes USA - Melbourne payload limited.
Of course Boeings don't always deliver either - Rolls Royce paid significant fuel penalties to Qantas for 10 years after they ordered RR powered B744s.
Common sense would suggest QF would be covering their options. Would have to be brave to cancel the A380 with its potential, but would be wise to have a fallback option if it proves a dud. Peter Gregg was recently quoted as saying there'd be another fleet announcement in 3 to 4 months