I thought I'd preemptively head off any questions ;-)
AU handsets do have a SIM-like "IC card" (RUIM), and it does in fact look exactly like a SIM card. However it does not serve the exact same purpose in the case of an AU handset. When KDDI presented arguments against SIM unlocking, they pointed out that simply switching the RUIM card between two handsets would not allow two AU customers to swap phones. This is because AU CDMA-2000 handsets are internally registered to both the network and the subscriber.
Unless something drastically changes, an AU iPhone will not be usable with US Verizon or Sprint, nor will the reverse be possible. (Roaming will be possible.) Also a softbank iPhone will not be usable on AU. Technically it is of course possible, but good luck getting AU to register it to their network.
In addition, the radio of the US version supports CDMA EV-DO Rev. A 800, 1900 MHz, while the Japanese version has 800 and 2100 MHz, the frequencies AU uses.
Quoting Apple's tech specs page in English.
CDMA available only if iPhone 4S is sold and activated for use on a CDMA network.And Japanese
CDMAは、iPhone 4SをCDMAネットワークで使用する目的で購入およびアクティベートした場合にのみご利用いただけます。So, now that we got that out of the way, the more important question is: Will there be a region-specific SIM lock on an AU iPhone to prevent you from taking it to a domestic WCDMA carrier or MVNO like softbank or bmobile? Remember the debacle surrounding the original Japanese iPad? (Honestly, I never figured out WTF was going on with the original iPad baseband and I didn't care enough to exert the effort to dig through all the smoke and mirrors, of which there was a lot.)