iPad is hitting shelves tomorrow but iPad apps are already live in the iTunes App Store. As told on the iPad announcement day that all iPhone apps will be supported with iPad OTB but the apps for iPad are developed and optimized specifically for iPad and offers bigger resolution and works perfectly on iPad.
You can purchase and download iPad apps available in the App Store even thought the iPad is not yet out. After downloading, iPad apps will appear under separate section in the Applications pane below the iPhone and iPod Touch applications section.
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