People who keep bringing up the tech cap seem to be forgetting that it was in use as early as the BCs. The Chinese Emperor Qin Shi Huang (famous for the terracotta army) drank mercury to prolong his life, and is believed to have died from it. His tomb had a map of china in it with the bodies of water using mercury. So much mercury was used in its construction, that his tomb has yet to be excavated, due to the high levels of mercury that persist to this day. He died in 210 BC. If aluminum is in DF, then I see no reason why mercury should not be. It seems like the sort of crazy thing nobles might mandate (hopefully so they drink it) Also, tombs with underground moats of mercury guarded by mummy soldiers and loaded traps would be awesome.
*edit* Wikipedia gives even more examples than I was aware of: Mercury has been found in Egyptian tombs as early as 1500BC. It was used as medicine by the Egyptians, Greeks, Romans, Chinese, the Maya, and in Medieval Europe. There is Plenty of precedent here people. I for one would love to have mercury available in the game.
Furthermore, if alchemy makes a comeback into the game, mercury would be an obvious candidate for inclusion, being one of the materials used frequently in alchemy.