I recently restored my HD from Time Machine, and it looks like everything in keychain copied over fine - except for one password for a very important disk image! When I look at my old login.keychain passwords, it's not there either, even though I mounted that image a hundred times from the keychain-saved password. Any idea where it went, or where it was stored?
Figured out the answer. To anyone wondering: the login.keychain file apparently doesn't have all the passwords in it. I had to copy the entire 'Keychains' directory over to my new system for them to show up. Disaster averted!