The guilt of ideas by association

Over at The Renaissance Mathematicus, thonyc is asking some very pointed questions about science - but the questions are really relevant to everyone.  First, how does a person's personal ethics affect their professional authority as seen by others?  Second, if your work might build upon the work of someone of questionable ethics, do you eschew acknowledging their professional contributions (thereby causing your own work to suffer) or do you refer to their work as you would anyone else's (at risk of legitimizing their less wholesome aspects)?  The case in point is a historian who is a leading expert on Galileo and also a Holocaust denier.  I'll be following the conversation.