how do I disable preview autosave? Today I was scrolling through my photography gallery with Preview app. I was playing around with files rotating them etc, only to discover that it was actually saving those files (overwriting originals) without giving me a notice!

Is there any way to make it ask me for changes everytime? not only when file is locked...
Are you using Lion?  If you are, you can check your versions by clicking the file title on the top of the window, then select Browser all Versions to revert back to the previous save.