And is your answer to merely go to a pc to "unarchive" them? Or did you figure out a way to do it on your phone (as you asked in your original question) and then just didn't want to share your discovery with the rest of us?
Actually you bring up menus, select "View Labels", then you may select "All Mail". To go back there will be an "In Box" label to select from.
This wasn't obvious to me either. "All Mail" is not available from the menu list. Its hidden under "View Labels" which really means view folders. At least the messages weren't deleted but eventually All Mail is going to get really really hard to view on the Droid.
While I was viewing an archived letter the menu let me change the label to personal but I could not find a way to add new labels. That will be my next search.