The new version of OSX, Lion, introduces Auto Save. It is a long awaited feature, but the difference Apple made to it is that it is not for each separate application, but covers now anything you do on your Mac, including Pages.
It can be activated or disactivated. And it allows reverting to the previous version of your project. Added to AutoSave are Duplicate and Lock functions. Duplicate turns your existing document into a template, and Lock is a clever feature that prevents accidental unwanted changes to a finished project. That happens! Open an old file to have a quick look and delete a few words or an image without noticing, lock stops it.
See full description on Apple site.
Read my objections to Auto Save in this previous article: 'Say No to Autosave'. For most of the users Autosave is a blessing rather than a threat, but be careful with what you write if you are on a local network.
And see 'Zap To And Fro: Undo-Redo Shortcuts' for why autosave and undo is needed.
UPDATE: German macusers discuss my arguments against autosave on the macfix forum. Join in there or comment here. Thanks!