What should potential employers know about your diagnosis, and when should they know this?

Often, clients have a fear how to appropriately express their conditions/diagnosis/uniqueness to possible employers, worried that once they know this, an employer won't value them as highly or make them do work that isn't exactly the kind of work the client wants to do.  This is a highly personal thing, but as the years have gone by and the general awareness and understanding of the Autism Spectrum has grown, I do sense a better climate for uniquely-abled workers to be forthright about what they are good at and not so good at, and to be clear about the kind of accommodations that they would benefit from at work.  Also, if you want to get technical and legal about it, the Americans with Disabilities Act and the amendments passed in 2009 mandate that employers make these accommodations for workers.  This combination of the legal requirements and the general awareness and acceptance of diagnosed conditions certainly have made the climate more conducive to people on the Spectrum being more open about their needs when looking for work than in decades past.

Practicing being clear and concise about what you have been diagnosed with is absolutely necessary; for one big thing, having "the speech" down and ready for use rather than fumbling with a million thoughts will show the person you are speaking with a small example of you dealing with your condition in a positive, businesslike way.  That said, I'm not sure I'd start the conversation with this topic, let it come up as the interview progresses, perhaps after they ask you (as an interviewer always does!) if you have any questions for them.  "Yes, I do" you might say, "Have you had much experience with people with Asperger's Syndrome (or whatever your condition is) in the past?"  See how they answer, and be ready for a quick follow up such as, "Well, I have Asperger's, and as such, sometimes I have trouble recognizing social cues, body language, and other non-verbal communication."  Let them respond, but also have the positive ready, like "However, with Asperger's, I also have tremendous attention to detail and deep focus, and will really dig in deeply to the tasks I am assigned and get you the kind of results that help the company!"  Again, practicing your conversational and informational bits will really stand you in good stead and help you be comfortable going in.

As for when to disclose, I think "the earlier the better," but would also include "the more comfortable the better," meaning be comfortable with the moment first, and that's hopefully sooner than later.  Some want to wait until there is a job offer on the table, and while that would give you great leverage, I think the person who has been evaluating you might feel a bit taken advantage of if you did it in that way.  Plus, I think you'd want to let them know about yourself and see how they react and see if they have had other workers that they have made accommodations for in the past, etc.  Look, they are hiring all of you, so let them know what "all of you" really means, and then when you do get the gig, you'll know that they are welcoming the entire package!