WebKit2 is described by Apple's Anders Carlsson in the mailing list message as:
Documentation concerning WebKit2 can be found on the project's Trac. Unfortunately, the open-source WebKit2 is not yet supported on Linux. Apple has just provided support for Windows and Mac OS X, but they will accept patches that add other OS support.
WebKit2 is currently considered a "technology preview" by Apple and has not yet landed in a released version of Safari. Soon Apple will be publishing a basic web browser demo that's powered by WebKit2 for demonstration purposes.