I would definitely approach someone who wears such a shirt. It's interesting to see what the OP did about his plan, and if he did what he wanted to do, how it worked out. It's too bad that he might not post in this topic again.
There are pros and cons to coming out with your condition. I think it depends on how you choose to come out. Telling everyone is not a good idea, for sure. Wearing something subtle on a t-shirt is probably better than just the obvious "Asperger's". I have this t-shirt: http://www.cafepress.com/+dark_tshirt,116354338
Most people don't know what it means, I think. I don't wear it to school, except for one time last year for only one period. When I wore it, I've overheard one of my classmates say, "Oh, so that's why she's always like that!". But I don't think most people knew/noticed/cared.
However, I don't feel comfortable wearing it to school for now until I get diagnosed.
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Leading a double life and loving it (but exhausted).
Likely ADHD instead of what I've been diagnosed with before.