KDevDesigner is a port of Qt Designer (code taken from Qt 3.3 distribution) to KDE technologies:
1) XML GUI
2) KParts
3) KFileDialog
4) KIconLoader

KDevDesigner consists of a part and a shell. 
Part can be used in Konqueror and KDevelop to edit and view ui files and projects.
Shell loads a part with "in shell" argument so that the part can create projects and ui files.

Other important notes:
KDevDesignerPart is Read/Write part which ignores readonly mode.
Designer Editor plugins are not loaded.
Available in system designer plugins are loaded if possible (tested with Qt 3.3 version).

KDevDesignerPart integration:
KDevDesignerPart implements KInterfaceDesigner interface (defined in tdevelop/lib/external_interfaces).

WARNING:
If you want to modify this code, try to introduce as few modifications to original Qt Designer code as possible.
Write your own classes and wrappers instead. The goal is to ease syncronization between the port and original Qt Designer.
Feel free to modify kdevdesigner_part.h(cpp) though.

FUTURE:
Trolltech promises that Qt 4 will have easilly embeddable designer. If that happens, this port will be dropped for KDevelop 4.