A while ago released a Compass Overlay, which (hopefully) makes reading directions of the navigation control easier. Well this new version does the same, but its (mostly) correct for the Magnetic Direction!
Magnetic Compass correct link this time!!
Note: its positioned to fit the navigation control of Google Earth 4.3, not the separate compass, which has only recently begun working for me… Updates a few seconds after stopping moving.
Now have a way to determine the magnetic variation at a location have some other ideas for using this in Google Earth, eg magnetic bearing along a line, but this is a starting point. For example in the uk the variation is often too small to be really seen on the tiny control.
Update: Rather embarrassingly just noticed the link above was wrong! It was to a static compass. The link above has been updated, and it should now rotate! Sorry for confusion… (always wondered why this didnt get much traffic!)