The damage done by mercury to cell membranes etc. may be permanent (though I'm hopeful appropriate supplementation and the like can alleviate some of the effects in the long-term), it is at least theoretically possible to remove nearly all of the mercury sequestered in nervous tissue. Here is a link to collected comments by Andy Cutler on mercury detox. In the section headed "Chelation progress and non-progress-- when to stop" see topic 510 entitled "how long to chelate": Here he states that it could take up to 100 lipoic acid chelation cycles to clear accumulated mercury. That is hopeful in my opinion (and I'm not being cynical).

Index of Comments by Andy Cutler