Shadowform
Passive
Assume a Shadowform, increasing your Shadow damage by 15%, reducing all damage done to you by 15% and threat generated by 30%. However, you may not cast Holy spells while in this form except Cure Disease and Abolish Disease. Grants the periodic damage from your Shadow Word: Pain, Devouring Plague, and Vampiric Touch spells the ability to critically hit for 100% increased damage. Casting of a holy spell will remove you from shadowform for 3 seconds. During this time holy spells may be cast without resetting the 3 second cooldown.
Might add a slight passive increase to holy spell mana cost to the talent in order to offset any balance concerns.
Thought that was a nifty idea :P
(I know my avatar is druid - but I played an SP for 3 years, after my druid for 3 :P)