Rodney Buike has written a detailed article about the importance of Physical security. I often talk about this topic myself as it's often depricated at the expense of the more technical aspects of security such as wireless, cryptography, biometrics et al.
Click here to read Rodney's article.
***Updated with corrected link***
The article link just goes to Rodney's bio :S
Thanks for letting us know - I've amended the link now...