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!
I tried it out and it works like a charm. And it’s proof that you don’t even need a native app to place Google Voice calls from your iPhone. By blocking these apps, Apple and AT&T are simply making it a bit more inconvenient to use Google Voice.