Android OS versions 3.x and above (Honeycomb, Ice Cream Sandwich) actually have a setting that allows you to specify "Owner info" text that will show up when the device is locked.
There are also many 3rd-party locks-screen replacement apps that allow for much greater configuration flexibility than the stock one, including the ability to specify any text on the locks-screen.
For the "unlocked" state, you can always modify an image to include the "if found..." text, copy it onto the device's SD Card (or internal storage area), and set that as the wallpaper. You can use one of the myriad of free image editing software (e.g. Paint.NET) to add the text.