I use a Linksys WRT54GL router, with Tomatos Firmware on IT. I set manualy the IP on the Router, and define the name of the NAS there (actually, named DLINKNAS).
Under Windows XP, Vista and my iTouch, i can access the NAS with the Name (we call that the DNS name).
When the query are send, like http:\\dlinknas or \\dlinknas (to access the file system), the router redirect you to the IP of the NAS, like all normal router do.
I'm not sure if all router can define a DNS name on it, but i know this "open source" version can do it.