Apple just released the Google+ iPhone App this morning after finally approving it. It really didn't take long for Apple to approve the Google+ app, right at two weeks. Google+ is now supported on the iPhone 4 as well as the iPhone 3GS.
I had trouble finding it in the app store, just use this link to download the FREE Google+ App. I attempted using this link to access the Google+ App on my iPad and I just got an error, I'd assume the iPad Google+ App will soon follow!