It's their first native app for the iPhone, making it easier to browse popular, upcoming and recent stories for whatever topic you fancy. You can obviously Digg—and bury—stories in the app, and read comments too.
The Digg guys described their iPhone app on the blog:
"We wanted to make it easy to discover interesting content on your phone and to easily jump into the conversations around that content. You can browse lists of recent content, search for the topics you care about, and save stories to check out later."