I am noticing similar problems with the teleport tethers in my own SMP server. It seems they only "work" if I put myself within rendering distance of the chunk where the tether is placed. Once I log out, they stop working to a point, though I think the tether(s) in my immediate area where I log off are still active while I am not on (and nobody else is on), because I have logged in before to find items loaded into a chest while I was gone (and nobody else logged in during the time I was off).