The newest development release of GNOME Shell is version 2.27.2 and it brings support for Volumes (such as USB storage devices) in the Places area, a slow-motion rendering option, better glowing for running applications, search improvements, and code clean-ups. There's also bug fixing and other work that went into this release too. The release announcement for the new GNOME Shell release can be read on the GNOME.org mailing list.
Mutter is the window manager for GNOME 3.0+ and uses Clutter for its tool-kit to provide fancy OpenGL effects. The Mutter 2.27.4 release that just emerged has a major code clean-up, various fixes, and other changes. The new Mutter release announcement can also be read on the GNOME.org mailing list.
Look for the release of GNOME 2.28 on the 23rd of September. The release of GNOME 3.0 is expected to occur on the 31st of March in 2010.