Techcrunch has an interesting "rebuttal" up regarding Apple's response to the FCC over the rejection of Google Voice. I use the quotes because I think the rebuttal part itself is off-target, while the conclusion is fairly spot on. Worst things first:
[Apple's response] strongly suggests that the Google Voice app replaces much of the core Apple iPhone OS function. This certainly isn’t accurate, and we believe the statement is misleading. More details below, but in general the iPhone app is a very light touch and doesn’t interfere with any native iPhone apps at all.
The crux of their argument is that, while Google Voice provides separate voice dialing, voice mail, and SMS functionality outside Apple's built-in Phone and Messages apps, users are still free to use the built in apps. More specifically, that Google Voice only replaces these things when the Google Voice phone number is used.
Users, at least in part, are going to be replacing the AT&T number with the Google Voice number (likely the reason to get the Google Voice number for a segment of users). Ergo, they'll be replacing the built in Phone and SMS apps with the Google Voice app.
No big deal, though, right? Why should Apple care if people replace Phone and Messages with Google Voice?