So, my iPhone 4 just decided to restore itself
After 14 hours of research and experimentation I recovered everything.  Here's how:


  I used iPhone Explorer to back up all of my photos.
  
  I jailbroke with redsn0w.
  
  Explored the filesystem with iPhone Explorer, and tried to copy the
  whole root filesystem (which, for some unknown reason, silently missed
  quite a few files, but oh well).
  
  Saw that both sms.db and all my application data were still intact. 
  Danced with joy.
  
  Removed the ReleaseType -> Beta key in
  /System/Library/CoreServices/SystemVersion.plist
  
  Restarted it, set it up as a "new" iPhone, and all my sms, apps, etc
  were still there.  Hurrah.
  
  Set my PC's date to a week ago.
  
  I then backed up in iTunes, and restored to the latest beta.
  
  Photos, notes, sms were back on there, apps were all deleted but app
  data remains, so it's just a matter of re-downloading the apps.


I love a happy ending :)