The NSLU2 is $80 and works like a charm. Mine runs debian and serves DAAP, UPNP, and NFS. There are lots of howtos on the nslu2-linux wiki, making this a very good NAS to buy.

Glossary:
CIFS - Windows' file sharing protocol. Most NAS's will support it out-of-box. Nokia tablets can do it with extra software, but I haven't tried that.
NFS - network file system. Faster, better, unix way of doing it. Nokia provides the necessary kernel modules in a package you can apt-get install. Google for maemo nfs. You'll probably need to enable it on your NAS manually too. Again, google.
Daap - iTunes6 sharing protocol, readable by canola (and itunes7). Run mt-daapd on the NAS to serve it.
Upnp - Universal Plug-n-play. An increasingly popular Micros~1 protocol supported out-of-box by N800 file manager, media player, etc, and on 770/2006 using Nokia's "Media Streamer" app (and maybe other programs in os2007he too). Run gmediaserver on NAS to serve it.

re: "you could be talking German for all I know!" The difference is, google can teach you 'bout this stuff quicker than german, and we can help you here without using german but not without some TLAs

HTH