As Qt WebKit's future is going to be based on WebKit 2 and Qt Quick 2, the module should be split into two modules in Qt 5:
Qt WebKit: contains the WebKit2 based functionality. In Qt 5.0, this module would have a QML API, and no C++ API. This module can become a part of Qt Essentials.
Qt WebKit Widgets: an add-on module that contains Qt Widgets based functionality from Qt 4, based on WebKit1. It contains both the C++ API and the QML API from Qt 4. You would write QT += webkitwidgets in the .pro file to start using this module. The "include-all" directive would be #include <QtWebKitWidgets>.