One of our employees had a Lenovo ThinkPad T60 laptop that had the nifty "feature" where he could not boot into Windows without having the laptop plugged into power. In addition, when he removed power from the laptop, the system would hang. My testing was able to narrow that down further to where if the laptop was not plugged into external power AND there was no live Ethernet cable plugged into the NIC, these symptoms would arise. After updating drivers and the BIOS and checking the ThinkVantage Power Management settings, I found a setting in the driver advanced properties called "Deep Smart Power Down". The way this feature was intended to work is to save battery power when there is no active cable plugged into the NIC. Unfortunately, what usually happens is that the system locks up during the "hot-swap" remove session that Windows sees when DSPD runs. Disabling this setting resolves the problem quite nicely and everything is running off of battery like it's supposed to. [more]
http://www-307.ibm.com/pc/support/site.wss/document.do?sitestyle=lenovo&lndocid=MIGR-63677