Microsoft and Nintendo do not, for various reasons, want to release official applications on the iPhone or Android platforms. Sony, thankfully, has no such qualms.
This is the Official PlayStation App, which has been available since January for Sony fans living in Europe. It's only now become available in Australia, though (Australia being an SCEE market... don't ask), so it's only now that I'm getting a chance to take a good look at it!
For a free app, it's got pretty much everything you'd want in an official application. You can see your friends list, see what they're playing, compare trophies, check a master games list and read the PlayStation.Blog. It's also been optimised for iPhone 4 customers, making it look extra-swank.
About the only complaint I have is that it's so slow to load some features; the clip above was recorded using my home's Wi-Fi, but on the road and using 3G the "My PSN" section takes an eternity to load.
I'm not sure why this isn't also available in North America, but if you live in the US or Canada and can get this working, you should be fine as the only things that appear region-specific are the PlayStation.Blog news feeds, and most of those articles are the same worldwide regardless.
The Official PlayStation App is available for the iPhone, iPad and Android, and is free.
Official PlayStation App [iTunes]