I have a weapon on my wizard that converts a small portion of damage dealt to life. Does this work for spell damage I do, or only on melee hits?
It works for spell damage as well as melee damage - the game is very much focused on you using your spells moreso than your weapons.
My Witch Doctor favors equipment with this enchantment - high DPS attacks can keep him alive in even the most challenging areas.
The Diablo 3 Wikia states:
Life and Mana steal only work on Melee, and physical ranged attacks.
I believe this means that only abilities that do a percentage of your weapon damage count, which would line up with my experiences so far.