As of the latest PTR build we now have access to most of the set bonuses for the tier 8 gear that's going to drop in Ulduar, including the bonuses for both priest sets, shadow and healing. In case you haven't seen them before, the bonuses are as follows:
The parts listed in bold up above are new; these numbers are just placeholders in the PTR client right now, but Ghostcrawler has clarified that the intended values are somewhere along those lines. No word yet on what "is cancelled" is supposed to mean, as far as I know.
I'm not a big fan of either of the healing bonuses. The two-piece is okay, but even with PoH being targettable I don't think I'll be casting it enough to get a large amount of use out of the bonus (at least, I hope I won't; it's an exensive spell and if raid damage is huge all the time in Ulduar, I will be sad). The four-piece is nigh-useless for Holy priests (to which concern GC's answer is basically "learn to play"), but at least Disc will get a kick out of it. I definitely prefer the T7 healing bonuses over the T8 ones.
I'll let someone else analyze the Shadow bonuses more (I may pick up Shadow for dual-spec, but I haven't played it since level 50/patch 1.10), although I do feel the need to mention that the Shadow four-piece is basically free haste rating. Unlike a certain other priest T8 four-piece bonus I'm not mentioning.
Patch 3.1 brings us Ulduar, dual specs, significant changes to all the classes, and more! We've got you covered from top to bottom with our Guide to Patch 3.1.