We have a new version of the Google+ iOS app (v. 126.96.36.1994) that features resharing posts! Grab the update from the App Store as soon as it’s available (this can take a little time so please be patient): http://itunes.apple.com/us/app/google/id447119634?mt=8
Resharing is easy. Simply tap on a post (like you would to +1 it). At the bottom near the comment box, you’ll see a small arrow. Click it and you’ll see the option to “Share this post”. Just choose your audience, add a comment if you like, and hit “Post”.
You can find more info about this update in our release notes: http://www.google.com/support/mobile/bin/answer.py?answer=1620754
As always, thanks for your feedback on the apps. We’ll keep working to make the Google+ experience awesome no matter what device you’re on.