Telstra bigpond wireless USB dongle $200 at Mwave + cashback
I use one - not telstra - and they're a damn fine idea if your a slave to your machine. The only thing that becomes annoying is when you hit dead spots. Alternatively you could always just buy the new iphone and connect via it.
There are some support issues with the different USB dongles out there. The Hanwei's are generally easier to find information on when you have install issues. I've only yet managed to get one of these to 'autodetect' and work without stuffing around on a PC and even then it was only for a limited amount of time. They're been fine on the Mac once you have the providers software installed.
So far they seem to be proprietary - ie locked. You can interchange sim cards on the same network though. No sure how you can unlock them but you can take the sim and drop it into a phone - although it wont work to dial up. I imagine someone somewhere has an unlock mechanism.