A few weeks after third-party apps started filtering in, Google's launched its own Google Voice app for the iPhone, and it looks to have all the most important features: call and text support, push notifications, and voicemail access (including transcriptions). It's available right now -- US only at the moment -- on iPhones with iOS 3.1 or higher as long as you've got a valid Google Voice account set up... so get to it.
[Thanks to everyone who sent this in]
Note: For some reason, you need to search the App Store for "googlevoice," not "google voice." Weird, we know.