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I recently wrote a post about guard classes (having not found any information either): http://ajdotnet.wordpress.com/2009/08/01/posting-guards-guard-classes-explained/
I also published a respective Guard class implementation (feel free to use this code as is, or to adjust it to your needs): ajdotnet.wordpress.com/guard-class/
With regard to the relationship between Guard classes and Code Contract in .NET 4.0 (successor to Spec#), have a look at the following post: www.leading-edge-dev.de/?p=438
(sorry for the fragmented links, the site only permitted one link...)
HIH, AJ.NET