How reliable are stick shakers?
The Australian Transport Safety Bureau recently published a comprehensive research paper on reported stall warnings in high capacity aircraft - http://www.atsb.gov.au/publications/2012/ar-2012-172.aspx 

The report says that 30% of all stall warnings were false. Note that stall warnings aren't limited to the stick shaker.

The report says that of all reported stick shaker events, 16% were false warnings. Have a read of it, you'll find lots of interesting bits of information.