I've been using Google Voice as my primary phone number since March of 2009. Shortly after switching over, I read an article about Sean Kovacs, an iPhone developer in Florida who was creating a native iPhone application for Google Voice. After a few e-mails were exchanged, he agreed to let me be a beta tester for his app, GV Mobile.
It was a great solution, and I enjoyed using the app for about 4 months. Sadly, towards the end of the summer in 2009, Google & Apple had a falling out of sorts, which ultimately ended in the removal of all Google Voice apps from the App Store (not to mention an FCC inquiry of both companies).
Fast forward a year to September 2010 and Apple announced they were relaxing their App Store approval policies, ultimately paving the way for the return of Google Voice apps to the iPhone platform. Last weekend that finally happened, when Sean's new GV Mobile + was approved and went live on the App Store on Saturday evening.
For iPhone owners who also use Google Voice, GV Mobile + is currently the best solution. Although Google released a fairly useful web interface for Google Voice earlier this year, GV Mobile + is a more elegant implementation since it benefits from running as a native app on the device.
So why would you need a Google Voice app? Well, if you want your Google Voice number to show up as the caller ID when you place calls, you need to initiate that call via an app or the web interface. If you don't, your "old" cell phone number will appear on your contact's phone, leading to the inevitable question, "Hey, didn't you tell me you changed your phone number?" Another benefit is that you can see the transcripts of voicemails within GV Mobile +, making it unnecessary to actually listen to the audio version. And perhaps most useful of all, you can send and receive SMS messages directly from the app, usurping the need for a text messaging plan from AT&T.
You can find out more info about the app and follow its continued development at his website below.