I'm sure all you priests have realized by now that PW:S now toggles your cursor to select another target when attempting to cast on a target with Weakened Soul. Rather than returning an invalid target error like it always did before you're now 'locked in' to a spell that didn't even cast, unable to cast anything else until you right click.
It's a major pain the !@# and a very real nerf in GCD efficiency as is, but you no longer have to deal with it.
With using the following addon and macro combination you can continue using PW:S as normal.
First, download and install the addon below.

http://depositfiles.com/files/mdvrl1567

Second, make the following macro and bind it to your PW:S key.

#showtooltip
/cast Power Word: Shield
/click SpellStopTargetingButton

Third, grab a beer and mash that button all you want without your UI trying to get you killed by the Horde.