For an interesting look at the defense measures, see "Physical Security Devices for Computer Subsystems: A Survey of Attacks and Defences" http://www.atsec.com/downloads/pdf/phy_sec_dev.pdf

IIRC there's a chapter on it in "Security Engineering" by Ross Anderson, too: https://www.cl.cam.ac.uk/~rja14/book.html (whole 1st edition is at the bottom of the page)

Edit: The story of Chris Tarnovsky and his work in satellite TV smartcards is also a pretty good read - http://www.wired.com/politics/security/news/2008/05/tarnovsk...