Holy Form

According to several posts, Blizzard is also distributing Wrath of the Lich King files with the 2.4 patch to prevent massive downloads needed at the launch of the expansion. I consider this to be a max-tier holy talent at level 80.
This was an interesting take on the skill (from the comments in the link Aves provided)
Personally, it seems to gimicky to be a talent. Possibly a scripted buff a player can get in a boss fight.
Yes, except that (having seen the tooltip reconstruction from the file) HolyForm doesn't say anything about losing the ability to cast shadow spells. Makes me think it's more likely an enemy buff at this point than a player form equivalent to shadow form.
However, if it were a level 80 move, or a new 51 point holy talent, I do not see the problem with it being better than shadow form...
I didn't look at the spell details earlier, Cost: 40% of base Mana, so I imagine it would be more like a buff then Shadowform, similar to a Pain Suppression & Power Infusion combo but strictly Holy only.