Anyone hosting a WordPress 2.1.1 install should upgrade or immediately prevent access to certain queries to prevent an attack described here. If the server is running Apache with mod_security, simply update your httpd.conf with the following rules:

<IfModule mod_security.c>
SecFilterEngine On
SecFilterDefaultAction "deny,log,status:412"

# RULES: Prevent WordPress 2.1.1 attack
# http://wordpress.org/development/2007/03/upgrade-212/
SecFilter "ix="
SecFilter "iz="

[...]

</IfModule>

And then restart Apache. Note that while this is an effective temporary workaround, upgrading is recommended. Also, any install *other* than 2.1.1 is not effected.