You can do it without jailbreaking by installing http://www.dottunes.net/ on your mac. The software is free but you will have to pay (£10) to access it from your Touch but then you can access iTunes locally or even externally through Safari.
I can access my entire iTunes library from my Touch or from my Opera on the Wii. It even streams video!!! Starting to sound like a salesman now so I'll shut up.
Thanks for your reply PMB - And I have changed the name now.
I guess when you talk about itunes Store, this is for buying music over the internet, but my plan with the iPod Touch was to buy the Universal Dock with remote - connect some Logitech Z4 speakers to the Dock, and a LAN disk where all the data was stored on.
With this setup I would avoid a PC running 24-7, and still have a cool gagdet in the living room, and if I could get the iPod Touch to connect to a share on a LAN disk I would have 500 GB of mp3 files available through WIFI, and still 32GB of music with me on the road.
You have to have the computer, or else you have nothing to serve the files with. The iPod Touch does not contain a full iTunes program, so it cannot pull the files directly from the disk. You must have a computer attached to the disk.
I personally have Salling Clicker, and I connect to my mac mini which streams to my airtunes on my outdoor speakers. I can control iTunes wirelessly throughout my house.