iPhone 3GS, when will more stock arrive?

In a nutshell? No one knows. I have spoken to people at both Optus and Vodafone and they have no ETA, 32Gb models are filthering into stores in limited quanties and at a very slow pace, maybe very 3 weeks or so. 16Gb models have not been seen by many people since launch. I would hope stock would become widely available again with the next 2 months, but other then that, dont hold your breath.