Getting to this point wasn't easy to say to the least. Here's a brief recap of the saga. First, Apple pulled all Google Voice related apps from the App Store in July of 2009, which prompted an FCC inquiry. Apple responded by saying, in so may words, "Uh, we were just studying Google Voice." Google shot back with, "Uh, that's not true."
Since then, a mobile Web version of the service has been released, and it's quite clever, but it's not a native app. Several native apps, GV Mobile + (US$2.99) and GV Connect ($2.99) have made it to the App Store and are filling the niche until the official Google Voice app appears.
Perhaps the development guidelines that Apple released on September 9th allowed Google to identify just what they needed to do to win approval. If TechCrunch is right, we'll find out in a few weeks.