David Kierznowski of Operation n has discovered a serious flaw in the Akismet anti-spam plugin that comes by default with the latest version of WordPress (2.1.3).
It has not been confirmed as yet, but I believe this will affect all versions of the plugin. The vendor has been notified, and more information regarding the vulnerability will be released when a suitable fix has been released.
I know its painful, but its recommended that you disable the Akismet plugin immediately.
The vendor was notified: 14/05/07
Response received: 14/05/07
Fix received: 14/05/07
The Akismet v2.0.2 Download upgrade has been made to address these issues and may be downloaded here.