Argh! I had not tested it enough too: actually with the etc/fstab entry below, the non-root user can mount the share very conveniently, but then only root can unmount it!
//server/share /mnt/mountpoint smbfs defaults,noauto,user,guest,rw 0 0
I just tried your method too, removing the fstab entry, but the result is the same: can mount, cannot unmount!
Going to browse other threads... to see if there is a missed point