Since Google Voice apps are currently forbidden in the App Store, David’s solution works similar wonders for them:
I’ve put together a web app at http://www.ironicsans.com/gv that generates bookmarklets for anyone you want to call with Google Voice, allowing you to organize your contacts as Safari bookmarks. You can arrange them in folders, and then dialing from your bookmarks is as simple as dialing from your normal Contacts app: just tap to dial!
Setting up the first phone number takes a few more steps than I’d like, but after that the rest are easy. Maybe you won’t want to take the trouble to add everyone from your Address Book, but it’s easy to set up a “Google Voice Faves” folder with all your most-called people.
Just point your iPhone to ironicsans.com/gv to get started.
NOTE: No phone numbers or any other information are sent to my server. All the magic happens on your end. And this is in no way affiliated with or endorsed by Google.