InformationArchitects released a first version of an OmniGraffle template for folks designing iPad apps. You can download it here, but it is not complete.Make sure you keep and eye on the site, because they plan to update it as they’re working on our own designs.
If you know your way around Photoshop instead, you can use this iPad GUI .psd . The PSD was constructed using vectors, so it’s fully editable and scalable. You’ll notice there are a few new UI elements as compared to the iPhone interface. The workable screen design is formatted to 768×1024 so anything you design in the Photoshop file can easily be brought over to the SDK.