I always had shares automatically available in the file manager with my n800 and n810 by using the avahi services (also makes bluetooth devices automagially browsable)
I installed avahi in the n900 but haven't had any success yet...
There are some manual tweaks that need to be done after installing avahi (e.g. renaming some files in /etc/rc2.d so that avahi gets started after dbus) -- perhaps I am forgetting some of the tweaks....
I'll play with the working n810 to see if I can figure it out...