You need to be able to have iTunes treat your laptop's hard drive as a virtual iPod. The reason is simple. There's no way in hell my entire music library will ever fit on the hard drive of any laptop at any time in the immediate future. At no time in the iPod's existence has my iTunes storage strategy ever been anything but putting my media files on an external drive. I don't want to lug my external media drive everywhere I go, but I do want iTunes to be useful software for listening to music.
It's probably possible to do this by creating a partitition on your hard drive and then giving it all the usual "iPod_Control" magic files and folder names. I wonder if anybody's tried this yet. (Unfortunately, though, even if you did that, you'd still have the usual thing with iTunes where if you open it without your media drive attached it starts labelling files as missing.)
Note to self: add virtual iPods somehow. Fix iTunes.