Saturday's Guardian newspaper feature about Julian Assange contained an amusing quote from a friend from his university days (Assange studied mathematics amongst other subjects): "I've often heard it remarked in the press that Julian has some idiosyncrasies. The people who make such remarks tend not to have hung around mathematics departments very much."
It's perhaps not surprising that mathematicians are associated with eccentricity. We love to tell stories about the idiosyncrasies of Erdos and Godel. Alexander Masters' recent book about the mathematician Simon Norton, The Genius in my Basement is an outstanding reflection on the biographer's art which doesn't do much for the public image of mathematicians. On the other hand, fictional mathematicians who are leading characters in novels such as Iain Banks's The Steep Approach to Garbadale and Ann Lingard's The Embalmer's Book of Recipes are reasonably normal people.
Are mathematicians more eccentric than other creative people? I suspect not: I am sure one can find just as many writers, painters, composers, actors, ... Is it perhaps just the abstract nature of our subject, and the difficulty of talking about the technicalities to non-mathematicians (or even mathematicians who specialise in different areas) which results in a focus on eccentricity? We can't tell our non-mathematical friends about a mathematician's brilliant ideas so we end up talking about their amusing eccentricities.
I'm not sure how far I have convinced myself!