I haven't gotten an email that says I have official confirmation on the offer, but I have been told to book a hotel in DC (which I've already done) and to fill out a direct deposit form for NSF to get the stipend just before the program starts.
The email I got from the contact at the Australian Academy of Science had a letter of invitation and she said that she would be sending along registration forms and information on opening a bank account for living expenses.
I think once you get confirmation from your host country, then it's official, but I could be wrong.
