I'm in the same boat (5 week US trip coming up in Sept-Oct) and while I expect the AUD to go higher, I've decided to hedge my bets a bit and convert $1kAUD this week. I'm doing it through my credit union who give good exchange rates and charge minimal (or no) fees.
The way I see it is - yes, it'll probably go higher, but not enough to make me kick myself if I change some now (ie: it won't hit $1 before Sept!) If I didn't change any though, and the dollar did drop, then I'd be kicking myself. Hard.
FWIW, I made a few purchases on my AMEX/VISA over here this week, and (when the AUD was around $0.828-$0.83) the exchange rate was approx $0.809. Still, given the exchange rate given by money changers (terrible), there's not many other options.
I've withdrawn some sample funds from my Wizard MC too, to what the rate's like. If it's good I may just withdraw the money here rather than converting it back in Sydney.
Cheers,
- Febs.