Found this new offer on the Jetstar homepage this morning

The Jetstar Price Beat Guarantee

Very similar to the "Double the Difference Voucher Guarantee" in terms but offers to beat other airfares by 10% instead.

This promotion is valid for bookings made November 23rd 2008 to February 23rd 2009.

Both the promotions are running and you can pick which one you want to make use of (although the Double the Difference Voucher Guarantee is currently stated as running only until 31 December 2008)