Is there a list somewhere of the countries in which an unlocked iPhone will work with a prepaid sim?

I read on Fred Wilson's blog that it works in the UK (and presumably the rest of Western Europe). I am specifically wondering about Australia and Japan.

Can I buy an iPhone 4 in the USA, get it unlocked in Queens, and use it in Australia with a sim?

Edit - I'm mostly wondering if the radio in the US version of the phone supports the networks in .au and .jp.