Here is a new really cool app for all iPhones that are jailbroken :) is called GV Mobile. The app is available throw Cydia, there is no cost right now for the app but if you really like it you can donate to the developer. It clearly said it on the description that this is not a product of Google.
-Dial numbers via the iPhone address book or typing on the keypad
-Full SMS support (view historic, reply, send new)
-Retrieve and delete recent call history
-Playback and delete voice mails
-Take calls from different phones other than your iphone
-Enable or disable the phones that Google voice forwards calls to
-Add or delete phones that Google Voice forwards call to.
The app looks really cool i am going to give it a try :)