You can accomplish this through home sharing. You designate one of your computers to be the main library and set it up with iTunes and all your media. Set up home sharing on all your computers (all you have to do is choose the home sharing menu option and enter your iTunes credentials). Then all your other computers will have a listing in iTunes for the library on the "main" computer. It will be on the left side above where the playlists would be.

Add new media to the main computer and you can stream it to all the others this way.

Note that this is streaming only and does not create a replica library on each computer. I don't believe that is possible.