I have an 11 year old who was diagnosed PDD-NOS six years ago, and I've been wondering the same thing.
From what I've read from others, when your child notices that there's a difference, and is questioning you about the difference, that's probably the time to tell them. It helps to point out that this isn't a defect, but purely a difference. At the same time you tell him, people have suggested that you point out the other famous people who definitely have Asperger's (Dan Akroyd) and those who may have it (Bill Gates, Steven Spielberg) to show that there are successful people who also deal with this difference.
In our situation, I can point to an older brother who would definitely been diagnosed with Aspergers when he was younger, and two grandfathers who would meet the criteria.
I've decided not to say anything to him for now. I think he may already know, but I'm not sure to what extent, as he talks about other kids with autism, but doesn't include himself in that group. I hesitate to say anything, because with this particular child, I'm afraid he will use it as an excuse to relax -- to not try because "I have autism". I also don't want to make him feel different, and I also don't want him to be talking about it all the time with other people. If he came and asked me, though, I would come right out with it, and if he was having a really hard time socially at school, again, I would explain everything.
I just feel as if, currently, he has an idea of what is going on, and just doesn't really care.
Kris