WordPress 3.0.3 is a security update which has been released to fix up a security bug which affects only the users who have remote publishing enabled. It is disabled by default buy you might have enabled it if you use an mobile app like WordPress Official apps for Android, Blackberry, iPhone, Nokia or a desktop client like Windows Live Writer to write content on WordPress.
Unlike the previous upgrade WordPress 3.0.2 released recently, its severity is higher, so its highly recommended that you upgrade to WordPress 3.0.3 even if you are using any previous version of WordPress.
Remote publishing is disabled by default, but you may have enabled it to use a remote publishing client such as one of the WordPress mobile apps. You can check these settings on the “Settings > Writing” screen.
Download 3.0.3 or update automatically from the “Dashboard > Updates” screen in your site’s admin area.