I just got off the phone with Brad from E-MAG Ignitions. They've just posted a new service bulletin on their web site:
http://www.emagair.com/E-MAG%20Servi...lletin%203.pdf
A few people have been experiencing problems where the sensor magnet has become loose, including one unfortunate guy that damaged his Thorp T-18 during a dead stick landing. You may read about that in some e-mail floating around the net.
E-MAG is tooling up to do a free retro-fit for units returned to them. They will essentially be replacing the shaft with a new design that utilizes a key-way and roll pin to secure the magnet, belt and suspenders style.
The service bulletin has more info on how to determine if your unit is affected.