[UPDATE: This is unexpected: AT&T has announced that it will let VoIP applications like Skype make calls over its 3G network. Will this change Apple's behavior toward Google Voice? We'll have to wait and see.]

Apple's app approval process is getting more arbitrary by the day, but one thing is becoming clear: Most apps will get approved, unless they compete seriously with Apple.

Case in point: Apple’s schizophrenic treatment of Google Voice and Vonage’s new iPhone app. When Vonage announced that Apple had approved its iPhone app last month, we were somewhat surprised; The decision came on the heels of Apple’s rejection of the Google Voice iPhone app, which, like Vonage, would allow users to make phone calls on their iPhone through a VoIP account.

But there was precedent for the approval, along with an asterisk. The Skype iPhone app was approved many months ago, and it lets users make calls through their Skype account — but only with a WiFi connection. If you’re using 3G — the mobile data connection that makes it possible for the iPhone to provide the “Internet in your pocket” — you’re out of luck: Apple and AT&T don’t want apps to compete with their voice service.

But the Vonage app, which was released this week, adds a wrinkle to this. It, too, lets you make calls via WiFi. But more importantly, you can make calls via your cellular connection, using your own number. You still rack up minutes on your existing plan, but any international calls you make are routed through the Vonage network, letting you take advantage of much cheaper international

Confusing? Just a bit.

This functionality — using the Vonage app to replicate the iPhone’s own phone application, and giving users added features — is what got the Google Voice app blocked in the first place. So why would Apple block Google Voice, but not Vonage?

TechCrunch’s Michael Arrington, who’s beyond miffed at Apple’s approval of the Vonage app, chalks it up to Apple’s fear of Google as a growing mobile rival:

…we all know that the real reason Apple won’t let Google Voice through is that they are scared out of their mind that Android and Google Voice will eat their iPhone lunch over the long term. Apple can’t win the fight over the long term, but they sure are willing to say and do anything in the short term to stop the advance of Google.

Since Vonage isn’t competing with Google in other areas — like mobile OS’s and even desktop OS’s — Apple isn’t concerned with the Vonage app altering the iPhone’s “distinctive user experience.” It’s time for Apple to fess up: they blocked Google Voice and 3G Skype just because they fear the competition posed by those apps and their parent companies.

So go ahead, download Vonage and make some cheap international calls. Just don’t hold your breath waiting for Google Voice.