the question is when to change more money to US dollars for Botcon..
Instability in the middle east (oil prices going way up causing a blowout in inflation in developed countries), Japan facing a financial crisis (as much of the country grinds to a halt), and European countries still struggling with debt issues... my money is on our currency heading south as investors head back to 'safer' markets & their currencies.
But just like a home loan, I'd recommend converting half of your BotCon budget now to US$, and the other half closer to the convention, or as soon as any significant drop in the exchange rate happens. Even if the exchange rate improves, it isn't likely to improve a significant amount before BotCon for you to miss out on a huge amount of US$ (from the half you convert now).
For me, a significant amount of my spending will be on the keycard and credit card (so I don't carry around and lose a big wad of cash). So in my case, I just have to hope the rate doesn't drop too much before BotCon or during that week.
Better convert A$ to US$ if the rates are good enough than to gamble with rates at the time of Botcon.