The vapor is a danger if the pure mercury is heated. Not at room temperature.
Snopes says the hatter story is undetermined.
http://www.snopes.com/language/phrases/hatter.asp
Everyone who took high school chemistry in 1975 had a puddle of mercury in their hand at some point.
Compounds of mercury can be a serious problem, as with lead. That is not what you have here.
Consider the lead issue to give some perspective.
Most all chimney flashing here in MA is lead. It is washed regularly with distilled water (rain) which then dumps down into the ground around the house. Some people have rain barrels and use this "leaded" water on the vegetable garden. Some houses have lead pipes for the city fresh water coming into the house.
So, either metallic lead is not a serious hazard, or, we all have lead poisoning.


Lead flashing is not under attack because people would not put up with having the govt make them rip it out. The people who make brass faucets and electronics with solder are not so lucky, and the rest of us are stuck with unreliable solder joints.
I am not advocating having an open bowl of mercury as a decoration in your living room. A closed jar in the garage is unlikely to reach out and kill you.
Regards,
404