In blender 2.6, is there a brush or key that lets me restore a certain part of the sculpt geometry back to its original state?
I tried shift with draw and clay brushes + shift key held down but that only smooths it.
I know ctrl held down with brush activates the opposite effect of the brush that has two settings.
Does a restore brush or function exist?
I think the only way is to use the Undo function , CTRL + Z .
The amoung of undo is controlled in the global undo steps , modifiable in the User Preferences -> Editing tab.
But the more undo you will have, the more memory it will eat.
I think he is not looking for an undo.
He wants a brush that “relaxes” or “fills” the sculpt back to the shape the basemesh originally had, and only where you use the brush.
I don’t think there’s such a feature though.
As you model, use Ctrl + Z to undo modeling you have made that you really don’t like. If Ctrl + Z no longer work, Smooth Brush is the brush I go for.
Scultpris has an excellent and extremely usefull “Reduce Detail” brush that Blender has no equivalent to.
Maybe using Sculptris for sculpting could then be a solution as this brush seems very similar to what the OP would like.
I see. Well I hope that feature is added soon. 
Yea, undo isn’t what I’m looking for.