Google just announced it’ll fork Webkit for use with Chrome/Chromium as it has become too difficult to add features to it as Chrome uses quite a different multi-process architecture than other WebKit-based browsers:
When Chromium started, our goal was to change as little of WebKit as possible, easing integration with the WebKit codebase. With Blink we are excited to make large-scale architectural changes to the code, without having to worry about breaking other consumers of WebKit.
When Opera announced they’re dropping Presto in favor of Webkit, they actually meant Blink (src).
Related: What is/isn’t Webkit? →