As pointed out on the WordPress development blog, a cracker gained access to the wordpress.org servers and replaced the 2.1.1 download with a modified exploitable version. The exploitable download may have been on the site for three or four days!
It may be a good idea for the WordPress developers to sign their releases with a well known and trusted PGP key. This would allow people to verify that downloaded files are really what they should be!
This is a well-established practice used by other projects, for example by the Linux kernel.