"I regret that Dr. Perelman has declined to accept the medal," said John Ball, president of the International Mathematical Union, which is holding the convention. Ball said later that he had met with Perelman in St. Petersburg in June, told him he had won a Fields medal and urged him to accept it. But Perelman said he felt isolated from the mathematics community and refused the medal because "he does not want to be seen as its figurehead," Ball said, without offering further details.
On the other hand, he's practical about the rewards:
If his proof stands the test of time, Perelman will win all or part of the $1 million prize money. That prize should be announced in about two years. [...] Ball said he asked Perelman if he would accept that money. Perelman said that if he won, he would talk to the Clay institute.