[Duncan]

Thanks,

I found a file in ~/Library/Autosave Information but unfortunately it was an autosave of a test document I created after the one I really wanted. It seems there is only ever one backup file in that folder. I managed to restore the original Autosave Information from Time Machine, but it would be nice if each unnamed document could be given a unique temporary name so they don't overwrite each other.

Or even better, if Omnigraffle could automatically open the unnamed backup next time it starts up after a crash (like e.g. Textedit does) that would be more intuitive than poking around in ~/Library as well as preventing inadvertent overwriting by forcing you to notice unsaved unnamed documents from the last crash.