When is it a good idea to let an academic department (or a professor) know that I have Asperger's (or ADD)?
I agree with aeismail's answer, however I would just add that you might need be careful about the perception of your condition(s), especially the negative aspects. For instance, with the ADD, it's better to ask people to send written copies because you have a visual memory, because it's a constructive remark, rather than saying that you don't fully understand verbal explanation. Myself, I ask people to send emails about important things, and I clearly state that it's due to my great visual memory and my terrible auditive memory. 

In other words, I think it's a good idea to inform people that you have Asperger or ADD, especially those you work closely with, but it's more important to explain the positive aspects rather than just stating your condition.