The most reputable sources are of course the official press releases.
However, I decided to include other sources because they often offer other aspects which are not in the official press releases.
I think the MX article is right.
It says on the AMS website at
http://www.ams.org/dynamic_archive/home-news.html#fields-2006 "In announcing the medal awarded to Perelman, John Ball, president of the International Mathematical Union, stated that Perelman had declined to accept the medal. At a later press conference, Ball said that he had spoken personally and at length with Perelman about his decision to decline the medal. While Ball was disappointed that Perelman remained steadfast in refusing the award, he noted that it is Perelman's right to refuse the medal if he wishes and described their discussions as polite and pleasant. Ball also said that Perelman will be recorded as having been awarded a Fields Medal but as having declined to accept it."
Refusal to accept the medal is quite a different proposition than merely not being able to afford to go to Spain to get it.
So I think I'll keep the MX article rather than delete it.