We saw the US team offer up a guide on how to get MMS working on your iPhone with the 3.0 beta software a couple of weeks ago, but it wasn't all that useful to us Australians. Fortunately, the guys over at Whirlpool have all the necessary carrier files for the Australian networks in one handy Wiki page.Head on over to Whirlpool's iPhone 3.0 Wiki page to get started and download the relevant carrier file. As always, this is a do at your own risk type of adventure, but there are plenty of people to offer you support over at Whirlpool if you come across any major difficulties.
Alternatively, you could just wait another couple of months before the 3.0 software is officially released to the public. Your call.
[Whirlpool - Thanks Jennifer!]