GTK+ 2.22 is expected to be the last stable GTK+ 2.x series release before all future development efforts are focused on GTK+ 3. GTK+ 2.22 will be released at the same time that GTK+ 3.0 makes its premiere this September with GNOME 3.0.
As described in the GTK+ 2.21.0 release announcement by Matthias Clasen of Red Hat, "[GTK+ 2.22] will not receive major feature work beyond API additions that are required to facilitate porting to GTK+ 3." GTK+ 2.22 will also maintain source and binary compatibility with the existing GTK+ 2.20 series builds, but as the GTK+ API isn't yet finalized, the 2.21.0 version may not end up maintaining compatibility with the official 2.22 release.
Some of the talked about features for GTK+ 3.0 include better CSS-based theming support, full off-screen rendering, resolution independence / free scaling of user-interfaces, a simple transparency API for widgets, physics features in the UI, full MPX / multi-touch support, easier widget creation, and many other features for popular this tool-kit. Serious development of GTK+ 3.0 has now been going on for more than one year.