I am suspicious about the origins of the letter with its (probably) harmless white powder.
The letter was written in Bahasa, a fact that seems to be eluding most news writers. I would be interested to know how many Australians can write in Indonesian. And how many of those Australians literate enough to write in Bahasa would be moron enough to do this stunt.
I would be interested to know which groups in Indonesia, and Asia in general, would profit from a falling out between Australia and Indonesia.
I understand that "face" has enormous value in Asian societies. It might not be beyond belief that an Indonesian has sent this to his embassy as a response to the Australian reaction to the Corby case. "You slander my country and call us names, now we can slander your country and call you terrorists".
Some interesting writings regarding Indonesia and biological weapons
here.