I remember having this same problem once upon a time. I forget how I solved it...

This[0]?

[0]: http://serverfault.com/questions/385920/disable-networked-ta...

I use shared connections, that makes tab completion scp usable.

  Host *
   ControlMaster auto
   ControlPath /var/tmp/ssh_mux_%h_%p_%r
   ControlPersist 4h