Thanks for asking, Steve.
Well, we've bought flight tickets all the way to Sydney and back and rented a car for the whole stay (15 days) in Australia. That's it for the moment. All the rest is still pretty much open. We are thinking of driving from Sydney to Brisbane, then down to Melbourne, perhaps Adelaide and finally back to Sydney. No Perth, Northern territories or Alice Springs this time.
I'll probably book a hotel for the first 2-3 nights in Sydney beforehand, but from that on we just do the daily bookings over internet, depending how far we reach that specific day. I hope that is possible in Australia, at least this is how we normally travel in Europe. Have not made the list of must-sees yet, but we bought a DK travelguide for Australia. And then there are all the valuable ideas in the posts above.
I am usually too lazy to make a very exact planning, like to leave some room for sudden ideas. It still is comfortably far away, we will land in Sydney on March the 30th. So if you have some more good ideas, let it come