No, there isn't a file with an .xsf extension. The last couple of times I've tested it there are NO files in the .cab file. The backup file still works just fine - I just lose any work I've done on the "master" file, if you will.
As I stated above, I've done some testing. I'll do something and then save the file to try to figure out where/when it happens. I think I've narrowed it down that when I insert a picture, then save it, then try to open it I get the error. What could a simple picture do to the form that causes it to lose all it's data?