GV Mobile is an iPhone app that lets you leverage your Google Voice account to make phone calls and send SMS text messages from your iPhone. So if you already have a Google Voice account and are looking for a way to use it on your iPhone (and iPod touch with limited features) take a look at the GV Mobile app in the App Store. It has the following features:

- Dial numbers (domestic for free and international at a variable rate) via the iPhone’s address book or typing in on the keypad
- Send SMS messages
- Retrieve and delete recent call history
- Playback and delete voicemails
- Take calls from different phones other than your iPhone
- Add, delete, enable, or disable the phones that Google Voice forwards calls to

Note: GV Mobile is not official, meaning it isn’t developed by Google themselves. Look for an official Google Voice app for the iPhone sometime very soon, as Google today just announced their Google Voice app for Android and BlackBerry phones. If you want a Google Voice account, request for one by adding your email address to the Google Voice invitation request form and check your email for the invitation from Google.

Update: Apple has pulled the GV Mobile from the App Store, but you can download it from Cydia if you have a jailbroken iPhone.

Here is a video demo of the GV Mobile app on the iPhone

Use iPhone mods and hacks at your own risk! Installing 3rd Party iPhone apps requires you to jailbreak the iPhone before installing the native software and apps. You can use Cydia to do this. Browse the full list of all iPhone and iPod touch native 3rd party apps filed under the software category.

More info here