Plasma Workspaces and the KDE Platform work is now focused upon Plasma Workspaces 2 and KDE Frameworks 5, which is why only the applications and development platform are still advancing in the KDE4 world. The KDE 4.12 stack is the first major update since the KDE 4.11 debut six months ago.
KDE 4.12 provides personal information improvements and other PIM improvements, the KDE platform received numerous stability improvements, and other changes have taken place.
More details on the software updates for KDE 4.12 can be found via dot.kde.org. Meanwhile for early adopters and developers we're awaiting the release soon of a tech preview of KDE Frameworks 5 and Plasma Workspaces 2.