This sort of hint has been touched upon quite a bit, but I haven't seen it executed just as I wanted (and it still isn't as automated as I'd like), but here's my offering.
Problem: 70GB of iTunes content on an external HD, but only wanting to take around 30GB on the MBP's HD, whilst maintaining ratings, play counts, etc.
Solution: Synchronising the iTunes Music folders with Chronosync.
First, download Change File Label Color from Doug's Scripts, and install. In iTunes, gather together the tracks you want on your machine. I created a folder called Portable iTunes, and populated it with playlists that contain music I want to take with me (these being the playlist that contains all my iPod music, an ever-changing playlist of movies I want with me, and my recently added playlist. These are either standalones, or combined in a smart playlist that references those already existing playlists).
Select the Portable iTunes folder, and then select all tracks (this is why the folder is important; it makes it a lot easier to have access to all the tracks you want on your machine). From the scripts menu, choose Change File Label Color of Selected Tracks, and set the files a color of your choice, let's say grey.
Download Chronosync and set up a new synchronisation, with the source folder as your iTunes Music folder on your external HD, and the destination folder as the iTunes Music folder in ~/Music/iTunes/ (this folder is used as when your external HD with all your music on is unplugged, and iTunes is launched; iTunes will revert to the folder ~/Music/iTunes/ making the process a lot easier.) In Chronosync, under Rules, change the drop down menu to Advanced and add this rule: Finder label is Gray.
Sync the folders (you may want to perform the "trial synchronisation" first). I have mine set to Synchronise Deletions like a smart update. This method now ensures that the files are arranged in the same way as the external hard drive, so that when iTunes changes the music folder, the files will be in the same relative positions. You could then even save the sync in Chronosyc and then place that file in your ~/Music/iTunes/ folder, and use the prefpane Do Something When to open it when you mount your external drive.
From now on, you should be able to add or remove files from the selection to be copied by changing finder labels using the method described above, and keep a synced but smaller copy of your iTunes library on your portable machine.
One bonus to this method is that it even copies folders to your iTunes folder on the machine with no contents, so you can always see what artists/albums you have without the external hard drive or iTunes. Hope this helps someone.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070203085555284