Per Seeking Alpha:
Update: AT&T responded to this post with the following statements:
AT&T does not manage or approve applications for the App Store. We have received the letter and will, of course, respond to it.
Customers can use any compatible GSM phone on our network, not just the ones we’ve approved and sell. And they also can use apps we don’t approve. We don’t approve iPhone applications.
So, it would seem that Apple are the folk that rejected the GOOG Voice application. Though, the company who 'gains' the most by this rejection is ATT (hard to see the upside for Apple, unless the rejection was formally done by Apple, at the behest of ATT). It's great the FCC is getting into this and one or both of these organizations will, hopefully, change their evil ways.
Bottom line is that if you purchase a device (phone, computer, tablet etc), you should be the one who decides what should go on it. Perhaps, ATT/Apple should read the GOOG 'ten things Google has found to be true'.
If pressed for time, 'Don't be evil' should suffice.