the overnights seem to be pretty evenly distributed now - so 5-14 a month is typical for all bases, as for anything less, or in between, it would all come down to how you bid and a bit of luck... So that means that you could get none, and you could get max overnights - depending on what you bid for, and a bit of luck.
To be totally honest - everyone I know was somewhat dumbfounded as to why they initiated a BNE recruitment drive in the first place, given their long term intentions.
The cockup that is advertising and cancelling a recruitment day is something you should all get used to if Virgin Blue is where you want to be. No doubt they'd (HR/management) still be throwing high fives around like confetti at the thought of a last minute cancellation, as opposed to addressing why it happened in the first place.