How about doors (made out of, say, Reinforced stone) that only open with the player that placed them there opens it. Also, completely indestructible; comparable to a door version of a personal chest.
And while we're on the subject of indestructable things, perhaps near industructable blocks (like reinforced stone, but with the blast resistance of Obsidian (or bedrock) and can't be removed except by a the player who placed it using a wrench. Maybe crafted from a Diamond surrounded by 8 Obsidian = 8 Obsidiamond plating (prononced "Ob-sid-I'M-ond"). (I copyright that word btw. For use by Alblaka and his team only. :P)
What I'm getting at here is: advanced plating from which to create invincible (or highly secure) factories for SMP, and to seal nuclear reactors from the rest of your build. (Not that any of you will tempt fate with supercharging those bombs-to-be).