Apple’s App Store has marked a milestone for the company. The company said 15,000 original applications are available with 500 million total downloads to date.

Given that the App Store only opened for business in July 2008, that averages to roughly 83 million downloads per month. That’s a fairly stout benchmark against which other platforms will undoubtedly be measuring their own success.

In its infancy, the iPhone was bombarded with complaints that users couldn’t install custom applications. Now, the large number of programs now available in Apple’s walled garden may be more soothing than some customers admit, considering the less successful response to other phone-and-application duos such as Google’s Android platform, Research In Motion BlackBerry Storm and Samsung Instinct.

But there’s still nothing mainstream about the iPhone and iTouch’s programming language (Objective-C) or programming application (Cocoa Touch). Apple seems to recognize this and has taken steps to ensure that neither Objective-C nor Cocoa Touch linger too long in obscurity. Apple now devotes an entire section of its Web site to helping developers create apps for the iPhone. The Web site includes instructional videos, guidelines and code samples. Those interested in developing applications for the iPhone or iTouch can now purchase a license for under $100 and download the software developer’s kit (SDK) from Apple’s Website. The SDK includes a complete set of tools for developing, testing and debugging original applications.

Beyond developing the application, perhaps the greatest challenge independent Apple developers face is passing Apple’s stringent “iPhone Human Interface Guidelines.” Ultimately, Apple has control over what goes to market and what doesn’t. The guidelines state that every design must create a “consistent visual and behavioral experience across applications and the operating system.” The document, all 386 pages of it, expounds on what exactly that means for prospective applications. For those that do pass, Apple takes 30% of sales, and the developer 70%.