Apple has sold nearly 90 million iPhone OS devices worldwide, and recently announced it has sold more than a million iPad devices. Meanwhile, the iPad 3G remains completely sold out in the U.S. weeks after its release.

“One million iPads in 28 days — that’s less than half of the 74 days it took to achieve this milestone with iPhone,” said Apple CEO Steve Jobs. “Demand continues to exceed supply,” he claimed, “and we’re working hard to get this magical product into the hands of even more customers.”

Wired noted that nearly one out of every 300 Americans already owns one of these devices.

iPad users have downloaded over 12 million apps from the App Store and more than 1.5 million e-books from the iBookstore.

That isn’t just good news for Apple: with such a platform designing custom apps for the iPhone, iPod Touch and the iPad is a great business opportunity.

If you expect to drop a do nothing app out there and get reach though, think again. iPhone OS is the platform you want to be on, but don’t get fooled: the competition is very high and there’s no “cash cow app idea”, not anymore.