Apple has released Security Update 2011-005 for OS X Lion and Snow Leopard and can be downloaded via Software Update or direct from Apple's website. The update is very necessary as it addresses a security issue with fraudulent certificates. As said on Apple's website:

"An attacker with a privileged network position may intercept user credentials or other sensitive information"

"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."

For more info. Visit Apple's support site.