Actually, if you ingest mercury (the metal), it's not that bad for you. Basically goes right through. It's mercury vapor and various compounds that are dangerous. The ABC article doesn't have enough info to identify what's really in the coal ash.
FWIW I believe I've located the primary source for most of the ABC News coverage [1]. Its embedded in a scare mongering (I mean public awareness there) site which does not appear to be affiliated with ABC News. [2]
Under full disclosure rules I must share that I am not a chemist.
http://www.ncbi.nlm.nih.gov/pubmed/16973445