This is a screenshot of the PSD. Using a subtle blur effect and a semi-transparent gradient layer I created a realistic looking screen image simulation.
Just a quick free PSD to show off your app or website on a smart phone. Files and instructions can be downloaded here: http://www.filehosting.org/file/details/558032/Simple%20Smart%20Phone%20Screen%20Replace%20PSD.zip
Instructions and screenshots are included.
Here is a screenshot if you right-click the Screen Contents layer, which has been converted to a smart object. You can replace the contents of the smart object and it was automatically replace the image in the final version.
This is a screen shot of the Smart Layer. Its dimensions are exactly that of the smart phone screen in the background image.