set dynamic_measures, on
distcommand or the Measurement Wizard.
When a measurement is created between two objects and either object is moved, when this setting is turned on, the distance object will update itself to honor the new object/atom coordinates.
We tested this feature in PyMOL v1.3. It was well-received and so we fully implemented this: it works across objects, states; it also works for distances, angles, and dihedrals.
Fully dynamic measures (see demo [32MB MP4]).