As per the Apple iPhone Human Interface Guidelines I have created a Default.png image which is displayed while waiting for the application to load.
I created the image u
It appears there are two options to create a high resolution (980x640) launch image from a screen shot of your application:
Using an iPhone 4 you can either use xcode to capture the image via the organizer window or by taking a screen shot on the device (using the lock and home buttons). I have not been able to test this as I do not have access to an iPhone 4
Using the iPhone Simulator you can emulate the iPhone 4 by going to Hardware -> Device -> iPhone 4 and capture a screen shot. In order to capture the screen shot you can use the built in os x screen shot functionality (cmd-shift-4). Alternatively there is a free tool called iPhone-Simulator Cropper which does a good job.