It's an interesting question which I think there will be a lot of debate on.
As you mentioned, haste will likely remain the best stat as it is beneficial for both dots as well as all casts (including healing).
But I think there will be much debate over crit vs. mastery. Crit is arguably better because it will effect the burst of things like mind blast and mind spike, but if you're grabbing both proc talents then mastery is going to be VERY valuable because you will barely have to hard cast anything at all, allowing you to be very mobile and barely interruptible.
It almost feels like it may be beneficial to go for some kind of balance between all 3 stats. Since I will likely be going with the two proc talents, I think I'm going to go for my maximum acheivable haste break point, then find a level of mastery which I find comfortable enough that I pretty much never have to hard cast anything but vampiric touch (will probably depend greatly on what comp you're running, and you will need significantly less for bgs), and then throw everything else into crit.
If I'm right, I think this is great for shadow priest pvp - and a fantastic job by Blizzard. It means that there will be no cookie cutter stat priority for us, but rather to maximise performance each individual priest will have to find what works best for them based on their gear, playstyle and talents. This will be perfect from seperating the great priests, good priests and bad priests!