We've discussed crying before here, here, and here. I have cried at work, but not in a work-related context: I got choked up when a family member was dying, when I got in a personal disagreement with a good friend at work, and when I was feeling particularly homesick. I also once comforted a friend at work whose pet was sick. I did not feel I had been unprofessional in these situations - but it's clear from the conversations we've had in this area that I could have beenperceived as such.
What I did think was unprofessional was crying in a professional capacity, i.e. during oral argument, trial, negotiations, or any other representative capacity. Recently, I witnessed another attorney cry in court. The thing that made this attorney so emotional was another attorney attacking her. Both attorneys had strayed from the core issues of the case and were focused on one another's behavior, which they each deemed inappropriate. Regardless of whether or not the allegations of improper lawyering were warranted, both the initial attack and the teary reaction seemed to me to be entirerly unprofessional.