Although there were over 80 security/bug fixes in WordPress 2.5.1, there was one thing that crept up immediately following the release. According to numerous reports by individuals and a ticket filed in the WordPress Trac when a user resets their password, the password reset link received in the email does not work. The error message that is received looks like this “Sorry, that key does not appear to be valid.”
This bug has already been fixed and will be included in WordPress 2.5.2. However, if you are having issues with the reset link right now, you can read this post by Ryan McCue on how to Reset your WP Password Manually through phpMyAdmin.
[EDIT] Ryan has included the files which contain the patch that you can download here