First, I've been using Google Voice on my G1 and I'm loving it. If ATT folds under the pressure I'm hoping the FCC is bringing to bare, I'll love it even more on the iPhone. So what if I'm using GV on the iPhone? I will still love the huge app stores, the games, MobileMe integration (which I don't get on the G1), and other iPhone specific functions.
Second, GV serves to compliment MobileMe because let's face it, Apple needs to add more features to MobileMe. Free SMS would be great. Chatting would be even more awesome.
So, it's not a threat but GV creates problems for Apple. One, bad publicity. If you're aware of Apple's rejection of GV app, then you know what I'm talking about.
The other problem is that it highlights how behind the curve Apple is when it comes to cloud computing. And it's very very far behind. It's even behind Microsoft. That's how bad things are for Cupertino in this specific mobile battle.
Right now, it's not a big problem because very few folks will actually leave the iPhone because they want access to a GV app. In all likelihood, folks will abandon their iPhones on principle stemming from the GV app rejection.
But 2-3 years from now, Apple's minor GV problem today could become a huge one if it doesn't get going with a comprehensive cloud, VOIP, and other mobile agendas.