IMO, our contenders for the post-3.1 DPS throne are:
Deep Affliction
Deep Destruction
Meta/Ruin
Felguard/Emberstorm
Master Demonologist/Pyroclasm, or some similar imp/fire mid-demo spec
I can't see a resurgence of Demo/Destro Shadowbolt spam with a Succubus - there's nothing to recommend it over Meta/Ruin. Similarly, I can't see any build without Ruin being viable - particularly with the changes to Affliction. That cuts out Afflic/Destro hybrids such as the popular SM/FG levelling build.
It's generally accepted that Affliction (in the hands of an excellent player pulling out all of the stops) is the highest DPS spec currently - partly because of the Drain Soul's c/f, and partly because Affliction doesn't have any particular attachment to its Felhunter and can happily summon an Infernal or Doomguard instead.
Affliction gets hit hard by the current build on the PTR. I'm seeing maybe 500 DPS less on a dummy. The true DPS loss could be even higher than that if you consider that Pandemic doesn't show up very well on the dummy in Live, but seems to show up fine on the dummy on the PTR.
I think both Deep Destruction and Meta/Ruin will be popular - the former for replenishment, the latter for DP/ISB. My opinion is that Shadowpriests, Ret Paladins, Frost Mages, and Survival Hunters give replenishment "more naturally" than a Destruction lock (they all pick up the talent and cast the linked spell *anyway*, where a destruction lock would probably pass over ISL in a raid which already had replenishment). Meta/Ruin mobs provide DP and ISB more naturally than the alternatives, and so I expect they'll be in demand.
Contrary to the opinion of some other posters, I expect Demo/Destro hybrids (with or without the Felguard) to remain very strong. The loss of hit on Cataclysm is nowhere near as make-or-break as it's been made out to be. 3% hit is much cheaper to make up on gear than, say, the crit we gain from having access to Backlash, or the new Master Conjuror. I raid 10-man without a Shadowpriest/Balance Druid, and I expect to have no problem whatsoever hitting 446 Hit Rating on gear - probably even without using the glove enchant. In 25-man with a 3% hit raidbuff, 368 hit rating is fairly trivial to reach. Remember, hit on gear is cheap compared to basically anything else.
My prediction is that the majority of raiders without an existing preference (e.g. those who just enjoy Affliction) will gravitate towards Meta/Ruin for farming raids - mostly because a 2-minute CD, 36s Meta + Demon Charge + Immolation Aura + Shadowflame + Hellfire is probably the best AoE going.
However, Metamorphosis still only accounts for a ~6% increase in sustained DPS vs a single target. That's not a terribly good deal for four talent points (Non-Meta locks will probably skip Nemesis). Also, bear in mind that in a raid with an Elemental shaman, only your damage *without* Fel Armour (bug?), and only your damage over and above 2800 (Totem of Wrath gives 280) is "counted" for Demonic Pact.
My second prediction is that despite the spec looking a bit messy on the calculator now, Demo/Destro will deal the highest 1-target DPS. I can't see 51-destro making up for not having the awesome talents in tiers 5-8 of Demo. Demo/Destro also benefits from most of the important buffs (most notably Master Conjuror, and the availibility of both Decimation and Backlash) while avoiding the nerfs to Affliction.
I'll be speccing
FG/Ember at least to begin with.
I'm quite excited to be able to use Dual-Specs to compare two builds without having to respec!
I'm also very interested to see what they do with Molten Core and Molten Skin. Notice those [PH] tags in the names?