Mocking up an iPhone app can be pretty laborious at the best of times. There may be a ton of applications and resources to help you get it done, but when it comes down to it sometimes the best resource is a Photoshop file to help you get moving on those designs.
Geoff Teehan has put together an iPhone 4 GUI PSD (including Retina goodness) to help you get your design mockups off the ground and ready for production. The file is pretty large, weighing in at 62.7 MB, and you can download them by paying watchcha’ like for the files directly on Teehan’s website.
Go check them out. It’s a great way to get started with your designs.