With the new patch 3.2 a new set, tier 9 will be introduced. Here a preview how the set will look for an alliance priest

There are some rumors about the set bonus we will get with our nice new t9 items, but its very possible that they will change until we get 3.2.

T9 set bonus / healing

2: Increases the healing done by your Prayer of Mending spell by 20%.
4: Increases the shield from your Divine Aegis and the instant healing from your Empowered Renew by 10%.

The 2-item bonus would be a pretty nice boost to PoM, not sure about the 4 item bonus yet.