Don’t do it!
Currently mastery for discipline priests adds 2.5% additional absorption power to your shields and Divine Aegis and about 1.25% healing for holy (holy is more complicated but lets just run with that). I’ve just started the theorycrafting for 4.0.x and mastery is by far the weakest stat for healing priests.
If you want to be trendie when 4.0.x goes live intellect is the new black. In fact, although it’s early days yet, i’d say healing priests of both flavours will be basing gear selection primarily on how much intellect an item has on it. We will also be gemming and enchanting for intellect.
Things may change but that’s my preliminary finding. More theorycrafting to follow sometime.