If you have already upgraded to WordPress 2.1.1, your site might include code for letting someone else control your blog, that was added in a break in. There is a new release available that removes the malicious code. Make sure you download it and upgrade.
This applies only to the ones who have upgraded to 2.1.1, not to the ones who are still at 2.1. This will also not affect you if you use code directly from the SVN repository. And of course, this does not apply to WordPress.com users too.
There are some more changes a WordPress user will have to do as the WordPress team has taken measures to ensure such break ins do not repeat in future. Users with SVN access might need to reset their passwords. This is an unstable situation with the ones with 2.1.1, but I am sure the WordPress community will handle it.