I have this set up at home. (Windows XP/Vista/7)
All music resides on my file server on a drive called M:\. This drive is shared as \\fileserver\m\. The Itunes library file is located at M:\Music\Itunes\ituneslibrary.xml.
Every computer maps the network drive \\fileserver\m\ as M:\, thereby making the paths to the files the same on all systems.
Load iTunes on each machine. During startup press Shift (windows) or Option (mac) to select the library file. On each machine you'll select M:\Music\Itunes\ituneslibrary.xml
You can add media from any system by simply dropping it into the M:\Music folder (or wherever your tunes reside). It will not get added to itunes on its own (Thanks alot Apple!), so you need to use iTunes Library Updater (ILU). This will add all your songs to iTunes and remove songs that you've deleted behind iTunes' back.
Some limitations: When you modify a playlist or something on one computer, you need to close iTunes to save the library information. Say you open up iTunes on two machines at the same time, make changes on one and close, it will save that library. If you then close the other one it will save it's version of the itunes library (without your changes you made on the other one). So be careful with that one.
This really depends, but I don't think it can be done the way you want it.
The 'supported' way
You can disable iTunes from copying music to the iTunes folder for future imports by going iTunes > Preferences > Advanced > Uncheck "Copy files to iTunes Music folder when adding to library". This will add all future files to the library, but it will not copy them into the iTunes folder, so make sure you do no.
The downside to this is that iTunes will not be able to keep those folders organized, so you would have to put them into the folder structure you want BEFORE importing them.
The hacky way
After importing the items into your library, move them to your alternate location, then create a symbolic link to the new location of the file. You have to use terminal for this:
ln -s /new/file/location/Britney\ Spears ~/Music/iTunes/Britney\ Spears
Assuming you moved the Britney Spears to /new/file/location/. The \ is used to escape the space in Britney Spears. This is important.
I have not tested this myself, so I do not actually know if it will work. I will try it out later and update this post
After further pondering, I have hypothesised a modification of the first method that should let you keep your ratings. After unchecking "Copy files to iTunes Music folder when adding to library", move the songs with the lower ratings to the external drive. Now, when you try to play the moved tracks iTunes should complain about the missing tracks. When it does, point it to the files that you moved on to the external drive.
If this works, it would be a tedious, song-by-song process which may be impracticable for a large amount of songs. You could also try going one step further by rechecking "Copy files to iTunes Music folder when adding to library" and hope that does not copy the files you just moved back into the iTunes Music folder.
Best Answer
This answer is obsolete. myTuneSync has since been declared end-of-life and is no longer available to download from the developer. Please see the other answers in this question for up-to-date answers.
myTuneSync will synchronize files, metadata, and playlists between multiple iTunes libraries. It uses the iTunes APIs to update your library so you don't run the risk of corrupting your library file.
There is a 15 day free trial if you only need to synchronize once, and a 3-pack of licenses is only $30.