In a huge update release today, Apple issued updates for iOS (moving it to version 4.3, in advance of the release of the iPad 2, due on March 11 in the US), Safari (to version 5.0.4), and the Apple TV (to version 4.2). The iOS update is available for compatible devices when connected to iTunes. The Safari update is available via Software Update. And the Apple TV update is available from the Settings menu on that device.
The iOS 4.3 update fixes a whopping 60 flaws, most of which affect WebKit, the web page rendering framework. Full information about this update is available here.
The Safari update patches many of the same flaws, again, mostly for WebKit. Full details are available here.
The Apple TV update covers five flaws. Information is available here.