Clutter 1.5.6 was just released a little more than a week ago and it didn't bring anything too exciting besides deprecating some Clutter functions, adding new functions and features, and a few smaller performance improvements.
Clutter 1.5.8 is now out and this release isn't much more exciting leading up to Clutter 1.6, but there are a few changes to note. This newest Clutter tool-kit development release brings performance improvements to ClutterX11TexturePixmap (it's used for storing a texture that displays the contents of an X11 pixmap), a GLSL (GL Shading Language) generation back-end for materials that is now used by default with Clutter when rendering to OpenGL ES 2.0 (GLES2), OpenGL Frame Buffer Objects (FBOs) are now used for more efficient reading back of partial texture data, there's some memory leaks that have been plugged, and there's various other changes.
All of the Clutter 1.5 work will make its way into Clutter 1.6 that should be released as stable in early 2011 prior to the release of GNOME 3.0. At the same time, GNOME developers are busy at work finishing up GTK+ 3.0 with many advancements to this tool-kit, including becoming more X11 agnostic (and Wayland compatibility), potentially an HTML5 back-end for running GTK applications remotely in your web-browser, cleaner rendering, draws more with Cairo, various API improvements, and many other enhancements.