While TechCrunch doesn't offer any definitive proof that AT&T is behind the Google Voice blockage, they do cite GV Mobile developer Sean Kovacs' point that Phil Schiller himself, who you may remember from Apple events like Macworld '09 and WWDC, personally approved the GV Mobile app last April. In fact, Kovacs says that Schiller called him to apologize for the delays in getting the app approved.
Given that high-level support for the app, it's not hard to follow TechCrunch's logic and set sights on AT&T when looking for reasons for its removal. While few people would argue that Apple's app approval process is pretty broken right now, it's hard to believe that an application that was approved by Apple's VP of marketing was somehow magically unapproved because it "duplicates functionality."
As TJ said in his post, it's difficult to believe that Apple truly thinks users will be confused by an application that does the same thing as the built-in Phone app when they went out of their way to purchase and download it. Even setting that aside, it's not at all clear exactly what functionality is supposedly being duplicated. If it's the dialer, then that's a poor excuse; lots of other approved apps include dialers, including Skype. If it's the SMS functionality, again, poor excuse, as other apps provide SMS functionality as well.
It's true that Google Voice isn't necessarily as "scary" to AT&T as a true VoIP app like Skype, but some of the Google Voice functions that TechCrunch describes, like free SMS messages and cheaper long distance, certainly do sound like something that would rattle AT&T's cage.
Take into consideration that Skype was approved because it will only work over Wi-Fi, even though there's no technical reason it shouldn't run over 3G. Look at how crippled SlingPlayer is compared to the same app on other phones. Now, Google Voice is dead in the water on the iPhone. What we have is a very clear and disturbing pattern. Applications that provide innovative solutions to users, but which fall outside the bounds of what a traditionally-minded telecommunications company like AT&T considers acceptable, are being either hobbled by arbitrary restrictions like Skype and Sling or blasted out of existence like Google Voice.
Whether it's Apple's fault or AT&T's, it's getting tiresome to see innovative applications like Skype or Google Voice constantly kicked to the curb or kneecapped. Are we headed for an App Store with nothing in it except fart apps? Those, at the very least, don't duplicate any built-in iPhone functionality that I've heard of.
One more thing: who else thinks it's ironic that a company started by a couple of guys who spent lots of time phone phreaking in a garage in the 1970s now kowtows to AT&T and kills apps that help people make cheaper calls?