Last week, Apple served up its two billionth iPhone app.  More than 6 million apps are downloaded every day, and more than 125,000 developers are officially working with Apple on iPhone apps — with God only knows how many more working unofficially.

It's hard to believe that it's been barely two years since the iPhone was introduced in mid-2007. Like the iPod, it's quickly become shorthand for a whole new way of interacting with friends, customers and data.

The early apps tended toward fun: beer drinking, simulated farts, jokes and games.

But the fun is being dialed down in favor of business. What was recently a 50/50 split between business and personal apps is now 70/30 in favor of business, according to a recent study.

For businesses, a phone app is becoming what a Web site was 10 years ago: a basic entry point into a marketing arena they can't afford to be absent from.

As has so often been the case in the brief history of our wired world, the new device unleashed an amazing burst of creativity. More than 85,000 iPhone apps are available in Apple's online store, a number that jumps by thousands each month.

As companies consider iPhone apps, one question they'll need to answer is whether they should offer the app for free or charge the consumer for the download. A recent study found that most free iPhone apps are rarely used after the first day they're downloaded.

Delivering value
Looking at more than 30 million downloads, Pinch Media determined that only 20 percent of users use free apps again after the first day they're downloaded. Within a month, most users stop using the app altogether.

But if you're going to charge consumers $2, $5 or more for your app, it had better deliver value. And that means more money up front. Developing an iPhone app can be as cheap or as expensive as you make it, but it certainly wouldn't be hard to spend a solid five-figure sum to develop a quality app.

It will be interesting to watch the performance of a couple high-profile news apps being rolled out this month. CNN is charging $1.99 to download its news app, and News Corp. plans to start charging a weekly fee of up to $2 for its Wall Street Journal mobile apps.

As with everything else on the Web, the biggest challenge moving forward will be helping users find your information. As the universe of data expands, seemingly without end, getting consumers pointed to your specific app will be Job One.