The change to WebKitGTK+ actually happened a couple weeks ago but the Planet GNOME RSS feed has been a bit wonky lately so the news is only coming out today. Joone Hur, a Korean Linux developer working at Intel and specializing on WebKit development, added an experimental Clutter back-end to WebKitGTK+.
Clutter, the tool-kit also supported by Intel, now allows for some hardware acceleration with this GTK+ version of the WebKit rendering engine. This support is still considered experimental and requires setting an environment variable (WEBKIT_EXTRA_MODULES) when building the rendering library.
Right now this Clutter back-end allows for accelerating basic features of CSS 3D transofmrations and animations. However, there are some missing features and some of the work from the WebKit-Clutter port are being moved into the WebKitGTK+ code-base.
Joone views this back-end as important since the eventual GTK+ 4.0 tool-kit is planning to use Clutter or Cogl (the Clutter OpenGL back-end) as a GDK back-end for the next-generation tool-kit.
More details on this work can be found in Joone's blog post from late March. Just days ago, WebKitGTK+ 2.0 was released as well.