Gosh I hope this works. I'm getting freakin' crazy with iTunes not finding any of my files after I start it up with the drive disconnected -- even after I reconnect the drive. You either have to add everything again (losing all your ratings, playcounts, etc) or reattach files one by one in my experience.

URGENT! itunes and external hard drives - macosx.com:

I have found a slight 'work around' for this problem. If you move the iTunes Music Library (data) and xml files to the external drive and replace the ones in your ~/music/iTunes folder with aliases (or symbolic links) to the ones on the external drive, iTunes will give you a message that the Library could not be found or written to. This will remind you to turn on or plug in you external drive and won't change the location of the iTunes Library.