What is ideal water hardness for brewing coffee?
According to a discussion I had on this matter with a friend in the coffee processing industry, you should definitely consider yourself lucky for your naturally hard water.  We live in a region with very soft water and he pre-treats all of his coffee-making water with a bed of ground granite (as a physical substrate) and powdered gypsum $\ce{(CaSO4 *2H2O)}$.  For individual use he suggests mineral water, defined in Wikipedia as:  


  "The U.S. Food and Drug Administration classifies mineral water as
  water containing at least 250 parts per million total dissolved solids
  (TDS), originating from a geologically and physically protected
  underground water source."


The "taste testing" conferences attended by my friend have rigorously determined that DI water gives the "blandest" taste and water high in calcium and sulfur containing compounds to be the best.  

Unfortunately, the chemical basis for this is poorly understood.  It could certainly be pH related, and that my friend's treatment bed just raises the pH which in turn better extracts the desirable components of coffee relative to the undesirable.  The same could be true of the mineral water and hard water in general should have a higher pH than soft water.