QF got their 717's from the takeover of Impulse Airlines, a bankrupt loco that QF acquired for virtually nothing. The 717's operated for Qantas Link initially (independant airlines that operate QF flights in QF-painted aircraft - somebody correct me if I'm wrong), but have just transferred over to Jet Star, Qantas's own Loco which launched yesterday.
I believe that Jetstar will ditch the 717's next year when they have been replaced by A320's.
The number of seats in the JetStar 717's has increased by 11 compared to the configuration they used to fly in under Qantas. They's done this by reducing the seat pitch by 5 cm's - a fact that Virgin Blue are exploiting to the hilt.