I've just bought a new iBook, whcih both my wife & I will use. We'll also both be using our iMac from now on. I've come up with the following solution:
Copy the "Music" folder from the Admin account to the Shared directory under "Users". Update the permissions on the Shared directory, ensuring that "apply to enclosed items" is selected. Then simply create an alias to the Music directory, and copy it back to your user account (make sure it's simply called "Music"). Then login to the other user account, delete the existing music folder, and copy the alias from the shared directory, again making sure to call it "Music". Hey presto, shared iTunes library.
The beauty of this is that it works for photos (just do the same for the "Pictures" directory), and if you invest in a good sync product like Folders Synchronization ($40), you can keep the iTunes library up-to-date across all your Apples.
I just received the iBook this week, previously the iMac was my wife's & I used a PC, so I'm not a Mac expert or anything, and if I can set this up, I think your average mac user could too.
PS I'm finally getting rid of the PC now that the mac can do everything it could (& more) without running Virtual PC... Yaay!