Apple Security Update 2011-005 is recommended for all users and improves the security of Mac OS X.
Impact: An attacker with a privileged network position may intercept user credentials or other sensitive information
Description: Fraudulent certificates were issued by multiple certificate authorities operated by DigiNotar. This issue is addressed by removing DigiNotar from the list of trusted root certificates, from the list of Extended Validation (EV) certificate authorities, and by configuring default system trust settings so that DigiNotar's certificates, including those issued by other authorities, are not trusted.
Note that this is for 10.7.1. or lower. If you're a dev using a 10.7.2 seed for iCloud testing, this won't install. You'll have to remove (not just revoke trust) the certs manually.