Apple have responded to the FCC's questions explaining the reasons why the Google Voice app has not been approved. The bit I found most interesting was how Apple used Visual Voicemail as one of the many reasons to not approve the Google app.
The Google Voice application replaces Apple’s Visual Voicemail by routing calls through a separate Google Voice telephone number that stores any voicemail, preventing voicemail from being stored on the iPhone, i.e., disabling Apple’s Visual Voicemail.So Apple think Visual Voicemail is something worth challenging other companies on when those companies try to mess with Visual Voicemail and other features. Such a shame they don't apply the same logic to the networks who have exclusive contracts with Apple for the iPhone. Look at O2 Ireland who after launching the iPhone with Visual Voicemail in March 2008 have still not actually enabled Visual Voicemail and have in fact shelved the project. Then when iPhone 3.0 was launched back in June tethering with iPhones was added. O2 Ireland haven't figured out how to enable that either. Tethering works fine if you get the settings but O2 Ireland still do not officially support it. Why should they, they have an exclusive contract for one of the most popular phones on the market so why bother investing supporting the features of that phone. I guess Apple only need to protect the features of the iPhone when it's Google who change those features.