Google has already released an iPhone app for Google+.
The app lets you manage the circles to send the messages you want the right people. Check the Stream to get updates from your circles and see the comments from other users. It also lets you write messages to members of a group.
Although the application is useful, it still has bugs. As it does not work in iOS 5 beta 3 nor IOS 4, the app sometimes just crash as it happened to a few users.
By now Google has released new updates to correct certain bugs.
You can download the application on the following link: itunes.apple.com/us/app/google