Q: Why can't Dia rotate arbitrarily?
A: Neither X nor GTK has built-in support for rotating arbitrary shapes (text, ellipses etc). Work is being done on a Display Postscript interface that will allow arbitrary rotation. Work is also in progress on FreeType support, which would allow for rotation.
Having rotated text is only part of the problem. We also need infrastructure so that handles, connection points, bounding boxes etc are rotated along with the object. Help on this would be appreciated.