The ThinkPad system powers off immediately when docked to a ThinkPad Advanced Dock, ThinkPad Advanced Mini Dock, or Essential Port Replicator. This could occur if the ThinkPad system is warm-docked (ThinkPad system is in a suspended state) or hot-docked (ThinkPad system is in a powered-on state).

Lenovo has determined that the reason for the system powering off is due to an electrostatic discharge between the system and the docking station or port replicator. We have developed a field kit that consists of two conductive rubber caps. These caps, when correctly applied to the dock, provide a safe path for the electrostatic discharge.

http://www-307.ibm.com/pc/support/site.wss/document.do?sitestyle=lenovo&lndocid=MIGR-67382