I've had the exact same problem. Our files are on a server but the backups are local. Typically the last thing I do is save and close all and the next time I check the files they won't open and I get the 'Unexpected File Format' error. Then I go to my local backup directory and my backups are corrupt too. I've been keeping 5 levels of backup so I can usually recover something, though I often lose several hours of work.
I've sent some corrupt files to our VAR and they've forwarded them to SolidWorks Development. Our VAR has told us that this type of corruption is often caused by a bad network.
There have been recent posts on this problem and its various permutations. I honestly don't know if this is a SW issue, a SP issue, or a computer/network issue. I'd like to find out though.