Google promised that iPhone users would soon be getting a Google+ app of their own, and it's now here. Not surprisingly, it looks a lot like the Android app, including views of your main stream and nearby users, sharing via Circles, and support for the Huddle group messaging feature. It's not a universal app, though, so iPad users are still left out for the time being, as are those with an iPod touch, for yet reasons unknown.
Update: Google's Punit Soni notes that there was a small issue that initially caused a test version of the app to to be served from the App Store. If you grabbed the app as soon as it was available (within the first hour and 40 minutes, specifically), you'll likely have to re-download it to get the current version (126.96.36.1999).