WordPress 2.8.2 has been released. This affects both WordPress and WordPress MU. I recommend upgrading your current version since it contains a security fix.

WordPress 2.8.2 fixes an XSS vulnerability. Comment author URLs were not fully sanitized when displayed in the admin. This could be exploited to redirect you away from the admin to another site.

For more details, visit:
http://wordpress.org/development/2009/07/wordpress-2-8-2/

You can automatically upgrade WordPress within your control panel, or manually upgrade via:
http://wordpress.org/download/

WordPress MU download:

http://mu.wordpress.org/download/