you could....
get an external hard drive and make iTunes host your media library on it. Plug this external hard drive into your router so it works as a network drive.
Then get a media player that is capable of playing media off the network, configure it to read off the network drive and you are in business. This part can easily be done with a little standalone PC. Actually, if thats is what you use, that would take away the need to have an external hard drive, use the PC as a server and point your PC's iTunes to use the media library on it.
Anyway, this comes to mind...
http://www.i-store.com.au/product/?productid=14229... just add a hard drive or 2 in there. Locate the internet router next to it and plug it in... hey presto! network drive. Use iTunes to store its library on it and use a media server to pick up the network drive over wifi and play stuff. It will make playback on your own computer a little slower because it will be accessing things over the netwok, but its not that big a loss. I used to run this setup to share movies in the house with my housemates, never had a problem with buffering or whatever. Just use a good router/modem-router.
Best part of this... you can turn the computrer off and still access the media.