This week, Apple achieved what may be one of its most impressive milestones to date. In the past three years, it has approved 500,000 iOS applications for entry into the App Store.

The App Store, of course, is the world’s most popular ‘app store.Billions of iPhone, iPad and iPod Touch apps have been downloaded through it, generating billions of dollars in sales.

For developers hoping to hit the jackpot developing apps for smart phones and other portable devices, the App Store is almost always priority numero uno.

Nothing reflects this more powerfully than the fact that the App Store now has some 500,000 iOS apps.

Even more remarkable is the growth in apps: little more than a year ago, Apple had approved 225,000 iOS apps, making the trend clear: despite the relative maturity of the Store, developers are continuing to build more apps, and they’re doing so at an increasingly rapid pace.

Also impressive is the diversity of the apps they’re producing. Games, not surprisingly, account for the largest percentage of apps (15%), but e-books (14%), entertainment (11%), education (8%) and lifestyle (7%) also account for a sizable number of apps.

All told, the majority of iOS apps (63%) are paid, and the average price paid is $3.64. When looking at the number of apps, their makeup and the fact that the majority are paid, it’s clear that Google’s Android Market will have a hard time competing with the App Store despite the growing popularity of Android mobile devices.

But is the future of iOS apps really as bright as the numbers might make it appear? Certainly, clutter is a concern that Apple will eventually have to address, perhaps more aggressively.

For developers, 500,000 apps is a big number, and lots of apps compete against each other. While a handful of the most prominent and successful developers cash in, increased competition is likely going to make iOS a more costly platform to develop for going forward.

All of this said, so long as the iPhone and iPad maintain their respective positions in the smart phone and tablet markets, clutter and development costs will probably not stop the massive growth in app volume the App Store has seen over the past year.

That means it may not even be too far-fetched to speculate about the millionth iOS app making its grand entrance into the App Store as early as 2012.