The iPad isn't the only thing making its big debut this Saturday. Several hundred newly minted iPad applications, many of which have been painstakingly redesigned and rebuilt for the device, will also get their first airing.

Developers granted access to the iPad software developer kit have been forced to sign a strict nondisclosure agreement that forbids them from discussing the contents of the kit. But they were able to discuss the challenges of porting their existing applications.

Almost all iPhone apps will work on the iPad, either in a small window at the center of the screen or blown up to fill its view. But according to many of the developers Technology Review spoke to, redesigning an application to make the most of the big-screen, multitouch interface has been the main challenge.

"The biggest difference for us was that the iPad has entirely different user-interface requirements than the iPhone," says Craig Kemper, one half of Little White Bear Studios, which makes the iPhone game TanZen. His team was able to reuse 90 percent of the original code to make the iPad version of the game. But they had to reconsider how users would control on-screen game elements. The iPad interface encourages developers to use popovers and split views in order to keep the application's main screen visible at all times. TanZen will appear as an iPad-native version on the iTunes app store, alongside the iPhone and iPod Touch-compatible original, Kemper says.

"We saw the iPad as a way to relook at everything we initially created," says Stephen Lynch, CTO of Panelfly, which makes a mobile comics-viewing application for the iPhone, and now the iPad. "On a phone, [a list] is how everything's presented--and you can't screw up a list," says Lynch.

Panelfly's new iPad comic book app is much closer to the size of a printed comic. It also allows for a much "flatter" interface, one that doesn't require the user to drill many layers deep when navigating Panelfly's 1,300-comic library of titles.

Following Apple's lead, as revealed through demonstrations of the iPad's capabilities three months ago, developers have come to realize that a bigger screen fundamentally changes how users can interact with a touch-based device.

"The touch technology of the iPad is the same as the iPhone," says David Jones, who teamed up with designer Amy Burton to create Sudoku Real Edition for the iPad. While two-handed gestures are impractical on the iPhone's small screen, they're logical on the iPad, says Jones. "I don't know if the iPad has a limit on the number of points of contact [it can detect], it's more whether or not those points of contact start merging," he adds.

Jones and Burton created their own gestures for interacting with the game board, some of which drew inspiration from an existing tablet-based device, the Fujitsu T2010, which uses directional swipes for everyday activities like cut, copy, and paste.

"It was a lot of fun--it's a great SDK," says Jones, who adds that his background in programming Web applications using the Ruby on Rails framework made it easy to build for the iPad. Like other Apple programming frameworks, the iPad uses the Model View Controller software architecture, which allows developers to make changes to the data, application logic, and the interface of a program independently of one another.

The simplicity of the programming environment means that many iPad developers will be tempted to port their apps to Android or other platforms. The Panelfly team already has an Android version of their application in the works. "A lot of assets for iPhone and iPad we can pass through and carry over to the Android platform," says Lynch.