Photoshop CS3 Tutorial
How to Revert an Image in Photoshop
Undoing changes can be accomplished with the Edit > Undo command, History palette, and Edit > Revert. When you revert an image, you return the image to the last time it was saved.
Open a new blank Photoshop document (File > New).
Add some text to the document by selecting the Text tool, clicking the workspace and typing.
Save your document (File > Save).
Choose the Text tool and type to add another Text layer.
Choose File > Revert and the document is back to the point where it was saved.