Apple has allowed the Google Voice App on the iPhone. The Google Voice App is now available on the App Store.
Originally, it was submitted in 2009, but never made it into the App Store or on a non-cracked iPhone. When Apple created more transparency within the app review process, it a lot of formerly rejected apps from resurfacing--Google Voice included.
The Google Voice app is distributed for free, and comes with free text messaging to U.S. numbers, voicemail transcription and cheap international long distance rates.