We recently had some hardwood floors put down and the following is the
manufacturers recommendation for stain removal.  It does seem to work.
 
1. For general maintenance occasionally damp mop with water containing a
   table spoon of vinegar in a gallon of water.
 
2. For removal of water stains wipe with neat white vinegar.
 
On the new floor the stains come off very easily with vinegar which seems to
deter future use of that spot for several days afterwards.
 
We have an older hard wood floor in a different part of the house which had
some real long term stains in a couple of places and wiping with white
vinegar removed them.  Obviously, where they had stayed so long without
being attended to the surface polish had also gone and I did a retouch
afterwards with some stain and wax that matched the original wood.
[Posted in FML issue 1786]