A bug in development versions of WordPress is causing some comments to be incorrectly caught as spam. The problem is in WordPress, not Akismet, and there’s no way for Akismet to prevent it from happening.
Technically-minded users can read the specifics in the WordPress Trac system.
We expect to have a fix in WordPress core soon, and deployed to WordPress.com shortly after.
Please follow the link for complete details and progress.