Update
Google have now updated the app so it now works on iPad and iPod touch.
I can't install Google+ on my iPad or my iPod touch
If you try and find the Google+ App for iOS from an iPad or an iPod touch you will find that despite searching you are unable to find the app. If you click a link you will find that the link on the iPad and the iPod touch will only take you to the apps available from Google, but not the Google+ App.
If you find and download the app from iTunes, you will not be able to add or sync the app to an iPad or the iPod touch.
The current version of the Google+ App for iOS will only work on the iPhone as indicated in the app requirements.
Though no reason has been given by Google for this limitation, one theory is that the app requires GPS and this is only available on the iPhone. Though the iPad with 3G does have GPS, the app or the App Store is unable to differentiate between the iPad with 3G which does have 3G and the iPad with Wifi that doesn't, as a result the app does not work on the iPad at all.
Another theory is that a 2x scaled up version of the iPhone app wouldn't look very good on the iPad and Google will release an iPad version soon.