Generate iPhone and iPad screenshots including device frame
Launch the app in simulator. Then Press Cmd+Shift+4 followed by space and then click on the simulator to take the screenshot with the simulator frame.
if you hold the Option key while you shoot the image, it will save the screenshot without shadows around the beze
On the AppStore Resource Center you can find Apple Product images that can be used to promote your app. There's also some guidelines on how to use the resources correctly.
UPDATE: Guidelines apart, a friend found a service today that does exactly what you want: PlaceIt
If you launch the iPhone Simulator in XCode you can just capture that window on a Mac using the built-in application called Grab. Outputs to a number of your favorite graphics formats.