The Boeing twin fleet manager said (some time ago) that plans are to increase the B737 fleet to 65-70 aircraft and the B738 economics are superior to the B734 so he expected the fleet to end up all B738. The original B738 order, late 2001, was for 15 firm plus 60 options (at the significantly discounted price that the post 9/11 market offered). Thus the entire fleet could be standardised at discount prices. I believe that the options have slots on the production line.
At the same time, the Qantas group has been looking at standardising the group's single aisle fleet (up to 200 aircraft) and the media indicate that a decision is imminent. It is quite possible that the entire fleet of B737's could be traded in to Airbus and replaced by A320's. I personally don't think that this will happen - it may be better to wait and be a launch customer for an entirely new and significantly more efficient (and quieter) single aisle aircraft which both manufacturers have been proposing.
I didn't realise how imminent the decision was. If the B737 fleet does increase to 65-70 aircraft then it would be a net increase of 12-17 aircraft. That would allow some promotions but nowhere near what Jetstar will get.