Propagating a Signal Through Old Walls?

avjewe asks: "I have a wireless (802.11) network in my 100 year old house. The walls are thick plaster, with enough metal in them to block the signal quite effectively. The floors, however, pass the signal just fine. Does anyone have any experience or suggestions as to how one can propagate a signal through a maze of faraday cages? I recently added an omni-directional antenna which, as one would expect, boosted the signal where I already had one, but didn't help the dead spots."