With the likely introduction of Icecrown Citadel next week, we'll be asking the question of what spec will be the most effective once again. Since I'm new in my guild, I'll have to be careful about my experimentation, but I do plan to examine each of the major raid specs in a raid situation once 3.3 goes live.
Next week you can expect to see me running the Elitist Jerks cookie-cutter Unholy spec. You can review Consider's excellent description of the spec here. If there's too much info there, the biggest change is that Scourge Strike has been hugely buffed and the mechanics of the attack have been changed. In addition, glyph of disease will no longer refresh at the original attack power, rather, it will refresh at the current attack power, making it a much weaker glyph for non-aoe fights. As a result, I'll be going back to a 2-hander spec to get the most benefit from huge Scourge Strike crits. At any rate, over the next few weeks, stay posted, and I'll be reviewing the various 3.3 specs on live servers in 'real' raids soon.