According to blues:
"We would like to get rid of spellpower plate, but we don't have a great system yet to make loot work without it. We want Holy paladins to wear plate and we don't want them to be great at say damage or tanking without having to swap gear."
Solution:
New holy talent XXXX - Transforms attackpower to spellpower (reducing AP while increasing SP). Also enables mana from strength (to remove caster plate completely). + some other effect since this is just a "dead" talentpoint otherwise.
I have no idea what conversion factors should be used, but I imagine that it can't be that difficult to find out.
Fun? Perhaps not. But a solution to Blizzard's itemization problems.