No one quite knew what to expect in late January, prior to Apple CEO Steve Jobs taking the stage to unveil the iPad. But no one was as nervous as the developers who had already invested so much time into the iPhone OS platform.

Cnet has an in-depth look at the challenge facing iPhone OS developers in adapting their apps for the iPad. Sure, the biggest hurdle -- the fact that the iPad uses the same SDK they’ve already become accustomed to -- may be behind them, but challenges remain.

iPhone OS developers breathed a collective sigh of relief in late January when they realized that their existing iPhone and iPod touch apps would, for the most part, run just fine on the new iPad. So did customers, since they (presumably) won’t have to buy all those apps again for their new device. But, the bigger screen size has presented some challenges for developers -- how to best get their 3.1-inch apps designed for the iPhone and iPod touch into the roomier 9.7-inch display of the iPad?

"We're starting to work on a game we originally positioned as an iPhone app, and it died because of the screen size issue. Now it will be our next project," aimed for the iPad, explains Michael Groves, one half of the development team at Wandering Pig Studios, who currently has two apps in the App Store. Like most developers, Groves is excited about the extra real estate the iPad will afford his apps.

However, Griffin web & interaction designer Cameron Daigle explains that it won’t be as easy as just upscaling existing apps. "What those (developers) are going to find is that the iPad has five times as much screen space, and your little app is going to look funny on there," Daigle said. "It's going to be interesting to see how people grow their apps to fill that space. You'll see a lot of awkwardly sparse and awkwardly cluttered apps as people figure out how to use that space."

While users have the opportunity to simply double the size of their existing apps to fill the iPad screen, developers like Groves worry that the experience will not be a pleasant one. “So he has to basically rework his app from scratch to make it a decent experience on the iPad,” Cnet notes.

"Scaling up never looks good -- it doesn't look good in Photoshop, much less something you're interacting with," Daigle adds. "Apple is doing that to provide a little bit of a transitional period. But people are never going to be happy with scaling."

Of course, the devices may yet serve very different markets -- after all, the iPhone and iPod touch are more likely to be carried with users almost all the time (if not all the time), while the iPad may be infrequently used due to its larger size, particularly for users who chose to buy the cheaper Wi-Fi-only model.

"I think the iPhone/iPod touch has been a training ground of sorts to get people used to this interface and concepts," Daigle concludes. "I think we'll look back at when iPhone first came out, (when app design meant a) top bar, bottom bar, and space in the middle. Apple did that on purpose, releasing the smaller design (of the iPhone) first to get people used to it...If they had released iPad first people would have been overwhelmed."