Media Masters of the Universe
I came across a site called mediamaster while watching a podtech podcast the other day. Basically its an online site to store your music. There is a webapp, currently in Java, that allows you to upload your music and even supports direct import from iTunes if you use it. Then you can make playlist and do all the usual music management stuff. Then no matter where you are you can just lauch the player, in a browser, and get to all your music.
I have just started using it, uploaded a few files, and played with it on the PC. Seems to work okay with Linux so that is good and if nothing else acts as a nice backup of your music collection!
Now to the Internet Tablet but. The web player is written in flash but it does not seem to work with the current browsers on the n800 so that's a bit of a shame. There is however a mobile version of mediamaster, http://www.mediamaster.com/mobile/, that lists all your playlist. Click on one of the links and it loads it straight up into media player. So now I can play my entire music collection whereever I am, as long as I have wi-fi :-)
There is apparently a new version of the interface coming out in the next few weeks so this may all change but for now it looks okay.
The podcast that I watched is available here as well if you want: http://www.podtech.net/home/4429/the-future-of-home-entertainment-mediamaster