Security by obscurity is, in a nutshell, a violation of Kerckhoffs' Principle, which holds that a system should be secure because of its design, not because the design is unknown to an adversary. The basic premise of Kerckhoffs' Principle is that secrets don't remain secret for very long.

Debate by Jesper M. Johansson and Roger Grimes:
http://technet.microsoft.com/en-us/magazine/cc510319.aspx

Urs