I could be wrong on this but I thought the autosave only saved a temporary file and was used for recovery purposes if the program failed to close properly. When the document is closed properly without error, then the temporary file is deleted as part of the clean up process.
If you know where the temporary file is saved, and you know what extension Excel 2007 uses for that file, then you may be able to do an undelete with some of the free software that is out on the net. No guarantees this will work.