Of course, the biggest question players have asked so far is whether or not the achievements will be retroactive. The short answer is: yes. The longer answer is: most of them will be. Starting with Wrath, Blizzard is going to be tracking an astounding number of things about our characters, and obviously some of them haven't been tracked the whole time -- it'll be very hard to determine how many yetis, for example, that you've killed, and so something like that will likely not be retroactive. But it's easy to know, for example, whether you've turned in a quest or killed a raid boss, and so that stuff will almost assuredly be retroactive.
My guess is that Blizzard will tune some of the old world achievements to make them more easily checkable (i.e. instead of "Kill 1,000 Furbolgs," it'll be something like "Be Exalted with Timbermaw Hold" -- something they can very easily know about your character). In the future, we'll probably have a few more silly achievements (Blizzard is already working on implementing a few more in-depth stats). And the real question is how/if this will all be shown in the Armory -- if websites can determine guild progress just based on the gear characters are wearing, imagine what we'll be able to do with all of this achievement data.