There are no further releases of 3.1.x (i.e. a 3.1.3) planned at this time. There have been a couple of patches applied to the CDT 3.1.x branch (cdt_3_1) after the 3.1.2 build that you may want to pick up. These builds are available on our built page at http://download.eclipse.org/tools/cdt/builds/3.1.2/index.html.
CDT 4.0 was released in June 2007. It's plan is here.
CDT 4.0.x will be released along with the other Europa projects in the Fall Maintenance and Spring Maintenance releases.