The Field's medal, according to wikipedia, is the "noble prize" of mathematics, only awarded once every 4 years to 2, 3, or 4 mathemeticians. Anyway, this article from the New Yorker describes the reasons that Grigori Perelman refused it. One reason, as the article quotes Mikhail Gromov (another Russian mathmetician), "the ideal scientist does science and cares about nothing else." Another reason that Perelman refused it appears to be that he was upset with the mathematics community because they were not being "honest" and playing politics instead of doing math. By turning down the award Perelman (who lives with his mother and is unquestionably strange--he lets his fingernails grow without trimming them, for example) is giving up financial gain as well as an oppurtunity to be recognized for what he has done.
While he has been made fun of in some arenas, his reasoning appears to me to be pure and well-intentioned. The problem that he helped to solve, by the way, The Poincaré conjecture, has been unsolved for 106 years and is invaluable to the field of mathematics. It will likely help many other scientists solve problems in their own respective fields.
There's some quality about keeping your head down to your work that I respect a great deal. Keep doing what you do, Grigori. We'll try to stay out of your way.