Grace of Elune is the moonkin-specific proc that regenerates mana when dealing melee damage. The tooltip says that a moonkin has a chance on hit to regenerate mana based upon attack power, this translates into how much mana is regenerated per proc and not how often it will proc. The proc appears to be normalized for weapon speeds so that faster weapons do not give an advantage.
As of the release of the Burning Crusade, there are now druid specific weapons with attack power for each of the forms which should be hotkeyed for melee attack. The Earthwarden is a prime candidate because of the huge armor boost it gives a moonkin doing melee damage. If there is no chance of taking physical damage, then a higher druid AP weapon is better.