It's pretty easy to come up with that estimate. They simply plotted the data and used a process of polynomial regression to estimate when Android would reach one million apps.
Since it opened earlier than the Play Store (nee Android Market), the iOS App Store has always been ahead of Android's market in terms of sheer numbers. However, it's been predicted for some time -- and by various market analysts -- that Google's app market would eventually overtake the App Store, as the rate of app submissions in the Play Store has been higher.
According to some estimates, Google caught up with Apple in October, or got within spitting distance, close enough so that there wasn't enough difference to make an issue over it.
However, the number of apps doesn't equate into revenue. It's well-known that Android lags way behind iOS in terms of app revenue, with Distimo saying in December that Google Play is bringing in less than 1/4 the revenue of the App Store, daily.
That is still a considerable amount, though, at about $3.5 million daily in its 20 largest markets vs. App Store's $15 million daily. In addition, Distimo said that Google Play was growing revenue twice as fast as the App Store. It's not all good news, though, as despite that percentage advantage, the App Store continues to grow revenue faster on a sheer numeric basis.
We've always said, though, that it's not sheer numbers that make an app store, but whether or not that market has the major apps that others do (with technical limitations taken into account, of course). At that, Android has been equal to iOS for some time.