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.
WebKit, the rendering engine used by both Chrome and Safari, is currently undergoing major redevelopment in order to support per-tab processes and out-of-process plug-ins by default. In one smooth move, Apple will be able to bring Chrome-like speed and security to its Safari browser.
Don't be fooled by its rather grand-sounding name of "WebKit2," however. This is more of an update than an upgrade. Basically, WebKit is being split into UI Processes and Web Processes. Each tab will become a UI Process, and presumably, so will add-ons and extensions. This change will bring the usual benefits of stability, security, and speed-ups from multi-core processors. WebKit2 will also implement a non-blocking API that is "mostly platform agnostic," resulting in a more flexible browser and better cross-platform extension compatibility.
The new WebKit2 will operate a lot like Chrome does today, only in theory, faster. With the split-process logic injected at a much lower level, it wouldn't be a surprise to see Safari out-perform Chrome. It will be quite interesting to see whether Google moves to support WebKit2, or indeed builds it into their browser.
I can't help wonder why Google implemented the split-process logic in Chrome, rather than being the major exponent of WebKit2, though. A competitive edge doesn't make much sense when it's all open-source anyway.
The WebKit2 patches are due to hit at any moment now, but I don't know when we'll see a version of Safari -- or indeed, Chrome -- running the new layout engine.