Google launched official Google Voice app for iPhoneNovember 16th, 2010
Google has released an iPhone native app for its Google Voice service, which gives a better experience than using the HTML5 web app client on iPhone.
All major Google Voice features are available such as international calls, free text message for US numbers, Voicemail transcription, caller id support. In addition, the app will alert via push notifications when you receive a new voicemail or text message.
Google Voice for iPhone is currently US only and requires iPhone with iOS 3.1 and a valid Google Voice account.