What's the Hidden Rule? (added 2006-08-18)
A king has four "activation squares": up-and-to-the-right, right, left, down-and-to-the-left. A king can only move if at least one of its "activation squares" is occupied by another king of the same color.
Addendum (2006-08-18): This puzzle is the first one to have the "feedback" and "homepage" icons in the upper right.