I'm having a bit of a struggle finding a SIM card that will work. Paid on a plan for years ($30 p/m +calls) and finally got around to canceling this service thinking (always dangerous) that the SIM from my iphone would work (my old prepaid wouldn't)... I got a micro sim holder and it fits ok but wont register.
Telstra were pretty unhelpful (

The next pitfall in the cunning plan is that its a real drama to get the phone from the waterproof case and change the SIM/holder, I can see something breaking and soon (i did manage to pin them down to their warranty, apparently Apple will replace the iphone outside their (12mth) warranty if you whinge a lot and fork out around $275 so all wouldn't be lost).
Anyhow, perhaps another SIM on a $10 plan (which seems the cheapest?) is the way to go. I can't find anywhere a mention of prepaid options? Int roaming essential? Anyone using sat phone? Any suggestions?
(Just aside, as an emergency help device, I did notice that the older irridium phones are v cheap on ebay, another option for 2 way comms for the bush (cheap if the SIM thing doesnt cost the earth (and chat time is kept down)?)