Just hold down the option key when launching iTunes and you will be given the option to to use a different music library (or create a new one).
One of my machines has an incomplete version of my music library, and I want to scrap the old library and get a full library from another machine. I first tried replacing the ~/Music/iTunes folder with the full library, but that resulted in some funky behavior. Copying my full library ~/Music/iTunes new and switching iTunes to the new library by holding down the option key works like a charm.
