From tomorrow all Australian Issued credit and debit cards will require a PIN.
Overseas issued cards will still work with a signature, however, EFTPOS units will be progressively upgraded so that only a PIN can authorise a transaction.
You shouldn't have any issues for the next year or so but I would advise you (Particularly Americans) to get a PIN for your card if you plan to travel to Australia. (or Europe... or most places in the world)