What is the best enchant to focus on getting on weapon/gear for PvP as a holy paladin?
In Cataclysm, you're going to have two chief contenders, Heartsong and Hurricane.
Heartsong increases your Spirit, and Hurricane increases your haste rating. Given the climate of PvP, I think that Hurricane will be situationally better, simply because haste will always be useful, while you're unlikely to be guaranteed a situation to make full use of the spirit proc every time.
On the otherhand, the material requirements of Hurricane are greater, so if you are strapped for cash, you might want Heartsong in the meantime (especially if you're using the same weapon to raid with, not unlikely when Cataclysm first releases)