The Help menu in designer has "What's new Qt Designer", which shows this documentation page: http://doc.qt.nokia.com/4.7/qt4-designer.html

The text seems out of date. It compares the "new designer" to Qt 3. It says that "The current version of Qt Designer is near feature complete and can be used for many tasks. However, it is still under continuous development. This document will explain what is already in place."