Australians get something of a raw deal with mobile phone app stores -- Apple skips applications apparently at random, BlackBerry skipped Australia altogether -- so Optus' introduction of a dedicated mobile app store is good news, at least if you're already an Optus customer.
The Optus Mobile Application Store sells applications for a variety of handsets, including BlackBerry, Android, Windows Mobile, Symbian and Java models. (It's no surprise that the iPhone isn't part of the package, given Apple's take-no-prisoners approach to selling apps.) Applications that are purchased are added directly to your mobile bill, which is a relatively straightforward way of doing it.
I'm not armed with an Optus mobile, so I haven't checked how the app works in practice; if you do give it a whirl (text APP to 966 to get a link), tell us about it in the comments.