Will you be an achievement junkie?
Questions have started to be asked about achievements for past content. Tigole, from Blizzard, has started answering the questions and it looks like we will be given achievements for some of the old content, where possible. However, some of it was never tracked and if you want that achievement you may need to go back and do it again.
We did a pass and checked quests associated with dungeon boss kills. In an upcoming Beta build, you'll see that you'll get some dungeon/raid achievements granted retroactively. Not all dungeons and raids have quests associated with them so some players will still have to re-do the content. For example, there are no quests that validate the boss kills in Slave Pens or Blood Furnace.
Also, keep in mind that in cases such as Onyxia or Zul'jin, not every raid member got quest credit for the quest item drop.
We figured it was better to try to reward credit wherever possible.