Whether you want to tweak your user interface or you want to take some promotional images of your iOS application, it is often really helpful to take a screenshot of your iOS application which is running on the iOS simulator.  It isn’t a big deal to take a screenshot of a specific region in Mac OS X, simply use the shortcutCommand + Shift + 4. However, an easier way exists by opening your application in the iOS simulator, open the Edit menu, press Ctrl and magically the menu entry Copy Screen shows up. By selecting Copy Screen the current screen in the iOS simulator will be copied to your clipboard and you can insert it in your favorite image editing software.

Update

All problems are solved with the new iOS 5.0 SDK and its corresponding iOS simulator because you have now the option to save the screenshot instantly to your desktop.