With more than 3-4 targets, the only class/spec in the game that is close to unholy is arcane mage.
This is because you have three diseases that are fully buffed by talents that are spread almost instantly to all mobs in the area with pestilence.
You can aoe with unholy with a rolling disease spec, or the 'traditional' 3.2.2 spec discussed in the Elitist Jerks forums; it shouldn't make a dramatic difference.
For large trash packs like Onyxia's whelps, here's the plan.
1. Icy Touch > Plague Strike > Pestilence. Now all mobs will have frost fever, blood plague, and ebon plague including the magic damage debuff. This is important to do first rather than D&D, because now when you do D&D, your damage receives the ebon plague boost.
2. Death and Decay. All mobs are now receiving maximum aoe damage, all runes are on cooldown.
3. Death Coil.
Tab target, repeat.
That's it. DON'T blood boil. Why? Because you have to use blood runes, and you need one blood rune for pestilence, and the other Death & Decay.
But there's a cooldown on D&D you say? Yes, so if you time it right, you might get one blood boil in. And if you do, more power to you. But don't mess up the very basic IT > PS > Pest > D&D rotation. This simple little sequence is the reason unholy is so powerful, particularly with 4piece T9 bonus. If you get this down, your guild will have a hand of sac/bubble rotation ready, because it will be almost impossible for the add tank to keep all the whelps off you.
For mobs that have trash around them like Lord Jaraxxus when infernals are out or Anub'Arak, then rolling diseases have an additional benefit from allowing you to maintain a normal rotation on the boss with scourge strike and pestilence. With 2 or more adds and the boss, however, I'd give up a scourge strike and blood strike to Death and Decay - theorycrafting has shown it's a dps gain.
So that's it. For 3.2.2, we're hard to beat.
Consider and others are posting on 3.3, and I may discuss that here at some point, but for now, this is the way to go.