GTK+ uses so-called "Linux kernel" numbering, where odd numbered series like 1.3.x and 2.1.x are unstable development versions, and even numbered series like 2.0.x and 2.2.x are stable maintenance branches. It is not expected that we will break either source or binary compatibility before GTK+-3.0
GTK+ development is discussed on email@example.com. We use the bugzilla bug tracker to keep track of particular issues; there is a milestone for each future release, as well as a 'future' milestone, for things not yet assigned to a definite milestone. (The components of interest for GTK+ development are 'gtk+', 'glib', 'pango' and 'atk')
Recently, we've also started to do weekly team meetings on IRC. The meetings were initiated to discuss and organize the 2.4 release, but maybe we'll keep them afterwards. The logs of previous meetings are available here.
2.2 – GTK+ 2.2.0 was released Dec 20, 2002. The rest of the 2.2.x series will be maintenance releases.
2.4 – GTK+ 2.4 was released Mar 16, 2004 with features such as a better file selector and a replacement "combo box" widget. The rest of the 2.4.x series will be maintenance releases.
2.6 – GTK+ 2.6 was released December 16, 2004 with features such as commandline argument parsing in GLib, rotated text in Pango, an icon view widget and a progress cell renderer.
2.8 – GTK+ 2.8 was released August 16, 2005 with features such as Cairo support, RGBA visuals, editing and drag-and-drop in the icon view and improved grab handling.
2.10 – GTK+ 2.10 is planned to be released around May 2006. Possible features include printing support and better introspection.
3.0 – A 3.0 release would be the earliest point at which we would make a source and binary incompatible release. (If and when we do such a release, there will be support for parallel compilation and runtime environments, as there are for GTK+-1.2 and GTK+-2.x.)
Ideas for future development of GTK+
An unsorted list of more or less blue-sky ideas for future development of GTK+ can be found here.