The iPhone 3.0 upgrade seems to be live in Australia. Navigating to the Aussie software update page gives you the phrase "Updating is easy" and instructions on how to upgrade. However, the page is a bit broken for us - images aren't quite displaying right.
According to the site, it's as easy as connecting your iPhone and hitting "Check for Update". At the time of writing, the same text doesn't appear on the UK or US sites, and network O2 is saying on its site that the update will be available "this evening". We're still betting on 6pm.
Thanks to commenter Jason for letting us know.
Update: Many Australians are saying that the update hasn't made it to iTunes yet, despite the Aussie Apple page changing. However, Pocket-lint reader Dave from the USA has sent us the above screenshot. We can't vouch for its authenticity however, and there's nothing we've noticed on Twitter, so take this one with a pinch of salt.