This is an excellent topic!
I have always been very open about my CF for many reasons: I need to explain my cough, I'm not ashamed of it, I think it's good to educate people etc. There are times when i just don't feel like explaining it so I'll say "oh it's allergies" if someone asks me about my cough. When I do tell someone, I always preface it with "if you google it, you'll think I'm really really sick, which is not the case at all." I don't go around broadcasting it, but I would say that about 1/3 of the people I work with know (I work with about 50 people).
I have always been extremely open with my bosses because I want them to know why I won't work more than 40 hours (need to get enough rest, fit in treatments, and excersize). It did burn me at my last job however. About a month after I started there, I was sent to Houston to work on a project and ended up in the hospital with pneumonia for two days. Mind you, that is the second time in my life I have been in the hospital. But, after that, one of my coworkers overheard my boss say that I was "not a reliable engineer" because I have CF. I was so heartbroken I thought I was going to vomit when she told me this, and I quit not too long after that (for that and several other reasons).
Now I work at a place that is MUCH more understanding, I think in part because so many of my coworkers also have health issues. At first I was much more careful about who I told, but now that I've been there for 2 years and gotten a feel for the place, I've opened up more. I'd just rather people know why I have limitations instead of thinking that I am lazy <img src="i/expressions/face-icon-small-wink.gif" border="0">