To all those companies and developers focused exclusively on iPhone apps: Watch your back. The Android platform is catching up, and none too slowly.

As Android's growth continues to explode since the release of the Droid, only the most foolish of app shops are not planning to expand beyond Apple's walled garden. One developer, in fact, wrote that his app, which was showing modest, double-digit daily sales late last year, now reports that his app is making $13,000 a month.

When that kind of opportunity exists for a single app, why would developers put all their eggs in one basket, a.k.a. the "Jesus phone"?

A few weeks ago, we told you, "As of December 2009[...] 4 percent of all smartphone owners now use a phone running some version of the Android OS. That's an increase of 200 percent since the previous survey released in September.

"Respondents were also asked about their plans to purchase a smartphone in the future. Among those who planned to purchase within the next 90 days, 21 percent said they would now choose Android."

It's this growth that helped fuel the success of Eddie Kim's app, Car Locator.

In a blog post today, the developer revealed that his Android app "started as a little side-project while I was vacationing with my family, turned into a few extra bucks for lunch money every day[...] has continued its upward trend and is now beyond my wildest fantasy of what could have been possible. "

Car Locator is a pretty simple application: Users save their location when they park their cars, and the app navigates them back to their cars later. The app was available in free and paid versions with varying feature sets. The paid version originally sold for $1.99, and the price was later increased to $3.99. Kim has done no marketing for the app, but it did win third place in Google's Android Developer Challenge 2.

When Motorola's Droid was released, Kim saw his first major spike in sales:

"In the first 2 months, the app saw sales of about $5-6/day. Nothing too fancy," he wrote. "But starting November 7, there's been a significant uptick in sales, peaking on November 9, where the app saw $44 in sales. Sales have since settled to about $20/day, but it's probably too early to tell if this will hold."

Little did Kim realize that his sales had just begun. To date, the free app has been downloaded 70,000 times, with paid app sales at about 10 percent of that figure.

"The application was netting an average of about $80-$100/day, until it became a featured app on the Marketplace. Since then, sales have been phenomenal, netting an average of $435/day, with a one day record of $772 on Valentine's Day. Too bad I didn't have a Valentine's date this year - we would've gone somewhere real special!" (Catch that, ladies?)

Kim also stands by the Android platform, saying, "Some may be quick to point out that a featured Android application is only able to net $400/day, while top iPhone apps make thousands[...] However, I still think that Android is only a fraction of what it will eventually become. Each release of a new Android handset gets me excited, as it means a wider reach for the Marketplace."

Folks, if you've been longing for a much-hyped app to make its way to the Android Market, forward this article to the developers and marketers in charge. There's money to be made there, and the userbase is only getting bigger.