The Undo History periodically purges itself (based on the amount of memory you 
have set up, I suppose).  It would be nice, though, if GIMP recorded and saved 
the workflow steps that were used as a retrievable text report.  No image data 
would be saved, but notes that layers were added, tools were used (with their 
settings noted and on which layer they were used), filters and script-fus that 
were run are noted (again, settings, layers, etc.).  Basically, one would be 
able to review this log and get a good idea of what was done to the image.  
This log should be stored with the file for as long as desired by the user.  
Enough smarts to know that a visibility on and then immediately off, or 
something that is done and then immediately undone, accomplished nothing 
material and are not worth recording would be nice as well.