As any serious PvP Priest has already noticed, you can no longer Shadow Word: Death out of a CC effect in PvP if you have Power Word: Shield running on yourself.  I use a simple macro to circumvent this:

#showtooltip Shadow Word: Death

/cancelaura Power Word: Shield

/cast [target=focus, exists] Shadow Word: Death; Shadow Word: Death

I hope I typed it out correctly.  What it does is automatically cancels the power word: shield on myself and THEN casts SWD.  If there's a focus target, it will cast it on it -- if you do not have a focus target, it will cast it on your target.  I know this macro is pretty self-explanatory, but I felt like I should throw it out there since being able to SWD escape polymorph, blind and freezing traps is essential to playing a PvP Priest well.