There are no further releases of 3.0.x (i.e. a 3.0.3) planned at this time.
CDT 3.1 is planned to be released at the same time as Eclipse 3.2. Current plan items are here. This release of the Callisto simultaneous release that involves the Eclipse platform, the CDT, and eight other projects.
New draft plan here.
CDT 4.0 will be released in the summer 2007. This will be a major release that will feature better multi-language support, project creation templates, amongst other improvements.