I'm not ready to draw any conclusions. If people who are open about being on the spectrum seem happier, I would really want to establish that we're dealing with cause and effect, and that's difficult on an internet forum. And then there would still be the chicken-and-egg problem: are they happier because they are open about it or are they open about it because they are happier?
I recall reading a number of posts where people said they don't tell others (especially at work) because chances are overwhelming that they wouldn't take it well. If you already know that, you're likely already not being treated as well as you could be, and you may not be the happiest person as a result. Being open about an often misunderstood neurological difference may not improve your situation at all in such circumstances. With friends, I agree - if they can't cope with that knowledge, they weren't really friends. But you can't usually change or dump jobs the way you can change or dump your social circle if need be - you usually need to endure whatever consequence follows for at least a while longer. So, I can understand being guarded at work / in situations that you can't easily leave.
In the past, I actually had a boss who was quite taken with several of my Aspie/HFA traits: no interest in socialising with co-workers, really good at working on my own in solitude, great persistence and an eye for detail, to name the ones I can think of off the top of my head. But I remain convinced that all that would have changed if I had told him that I'm on the autism spectrum. The negative image of Asperger's/HFA would have outweighed the obvious advantages for that job in his mind - I was intimately acquainted with what he thought of and how he treated other people who had disclosed i.e. depression, ADHD etc.
Of course I would love to say 'Just be who you are and let the chips fall where they may', and I'm sure that's good advice for a number of situations. But many non-autistic people really aren't that well-informed and tolerant, and that's why I always run through a worst-case scenario before telling someone, and if I am not ready to deal with that outcome, I keep it to myself. I'm not ready to martyr myself for the cause at the moment.