Download: Safari 3.1.1 released
Safari 3.1.1 is now available for Windows and OS X. According to Apple, this update includes improvements in stability, compatibility and security. "
In terms of security, this security bulletin describing the changes. It seems as if there was a vulnerability that could allow malicious sites spoof URLs headers or execute arbitrary code. There are two issues that specifically affect the version of Safari, so users of both platforms should ensure upgrading.
Just a reminder - some plugins, as Saft, will stop working after installing the upgrade. Visit the plugin author of the homepage to download the latest versions as they become available.