There may be an easier way so if someone has one chime in, but this is what I do:
Open up Terminal, type in:
defaults write com.apple.finder AppleShowAllFiles -bool TRUE
killall Finder
Then plug your iPod in, (this may or may not work on a touch), I always have enabled disk mode on so I'm not sure if that makes a difference but anyway you should see your ipod on the desktop.
When you open the icon you will see a hidden file that says something like "Ipod control" (I forget the exact word) but in the folder you will see "Itunes music" and then all your music folders with weird names like "X49B". Drag and drop all of those folders into iTunes and then reauthorise your account.
After you do that then open up terminal again and type:
defaults write com.apple.finder AppleShowAllFiles -bool FALSE
killall Finder