The Unofficial Apple Weblog (TUAW) sums up fairly concisely just what is up with the status of Google Voice on the iPhone.
If Google GIVES the Nexus One to all GoogleVoice customers, with some kind of incentive to move to T-Mobile, it will initially be AT&T who suffers. Do you remember the old days when incentives were paid to consumers to "Switch" their long distance carrier by MCI and Sprint. This was back in the days of Alternative Long Distance Dialing. It ended up creeping along until Judge Greene decreed that equal access to Long Distance carriers had to apply to all Regional Bell Operating Companies. That led to divestiture. When you look at it, you don't REALLY have a choice of long distance providers you can have with your mobile phone ANYWHERE IN THE WORLD, unless you use an application that lets you dial around, call back, ring back, call through, etc. All those actually add seconds to your calling time, and without unlimited plans do eat into your bundle (think Pre-Paid users.)
My prediction is we will see the unbundling of Long Distance from your mobile plan, and the mobile operators will be forced to allow user to pick their long distance provider for calling. This will really help the international caller who uses their mobile phone.
Google Voice may end up being the martyr in the battle, but they will be remembered by those of us who do see and enjoy being a telecom revolutionary.