Apple today released version 6.0 of the Safari browser fixing over 100 separate vulnerabilities, the majority in the underlying webkit layout engine. Many of the vulnerabilities allow code execution when visiting a malcious website and are rated "critical".
Safari 6.0 is available only for Mac OS X 10.7 (Lion) and the new 10.8 Mountain Lion, also released today. We recommend installing the Safari update as soon as possible. Mountain Lion itself is an interesting update in terms of computer security and brings the Gatekeeper component which will give an incentive to users to only install software from well known sources through the Apple App Store. Rich Mogull from Securosis has a blog post on Gatekeeper under Mountain Lion with detailed information.
If you have not upgraded to Lion yet, you need to evaluate how long older versions should be in use in your organization as security update support has now stopped.