Masta I don't think you can get the Milestone on Optus which is the carrier that Rukas is with.
Ruk if you can hold out a few more weeks you might want to wait for the Sony XPERIA X10 which is launching on Optus in early May. -
Sony Ericsson Android-powered Xperia X10 to launch this month on all networks
If not, go with the Backflip IMO. It has slightly better spec and a better keyboard than the DEXT.
Both are currently running Android 1.5 but will be getting a 2.1 update in July.
If you're not locked in to Optus, Telstra have the HTC Desire which is one of the latest and greatest Android phones. I seem to remember you had some issues with Telstra before so maybe this is out of the question.
I personally quite like some things about the Backflip, but it is a mid-range phone and of course ideally you want a high-end. My advice, go to the a Optus store and see if you can play with the Backflip for 10 minutes or so and see how much you like it - might as well ask them about the XPERIA X10 while you're there.
masta and I are both obviously phonegeeks though, and I would wager that most people who have a Backflip or Dext, really enjoy them. The Motoblur interface is supposed to be very intuitive for social networking and such.