From the Phb: Heal cures stuff like Feeblemind - which is arguably a form of insanity/mental damage. Limited wish/wish/miracle/greater restoration could be more apt depending on the exact type of insanity?
There is a spell in the Psionics book that treats madness - Psychic Surgery possibly?
The D20 Call of Cthulu book is probably going to have a good definition of insanity and treatments. I havn't seen it yet, but I'd be suprised if it didn't
Yup...from the SRD spell entry for Insanity "Remove curse does not remove insanity. Greater restoration, heal, limited wish, miracle, or wish can restore the creature"
But if you're talking about insanity from loss of sanity points a la Cthluhu or Unearthed Arcana...I don't know...that may or may not be different.