After originally reporting on the introduction of WebKit2 back in April 2010, it looks like it will finally be rolled out with OS X Lion, in a new version of the Safari Web browser.
WebKit2, rather than being a whole new rendering engine, is a layer around WebKit that adds more stability, security and speed -- not entirely unlike the Google Chrome sandbox, which is also strapped onto a version of WebKit. The most exciting feature of WebKit2 is that it splits the browser UI and the rendered content into separate processes. It's possible that each tab will have its own process, too, like Chrome.
This is the first solid news of a Safari update since the minor revisions of desktop and iOS versions back in November. It also represents a major change for the browser, so we wouldn't be surprised if it is Safari 6, rather than 5, that ships with OS X Lion.