You've long been able to use the mobile-optimised version of Qantas' site to check your bookings and frequent flyer status. A recent update means you can now also use the site to book and pay for flights.
The option covers both domestic and international routes. As you'd expect, the process is a little fiddlier than using a desktop browser, but it worked fine during a test booking on my BlackBerry.
It's not a feature you'll use every day, but if you were on the road with just a smartphone and needed to book a ticket suddenly, this could be very handy, and would save you the booking fee involved if you try and book over the phone. Mobile site booking works in Australia, New Zealand, Singapore, the UK and the US.