peer certification

This week I had an interesting conversation with kanarip about certification. Certified professionals have value to companies as they have guaranteed knowledge about a certain field. The professional has taken exams that are given out by institutes that are trusted by the companies. Mostly these institutes are connected strongly to the field, like Sun does for Java and Cisco does for networking. But how do we know that a certificate obtained by a professional is really valid.

The exams are mostly written ones and there is no real personal review of the actual skill level of a professional. Review is only done with on the spot knowledge and generally about a the field at that moment in time. Computer languages like Java, Ruby, HTML and other change over time. So do the written certificates really represent the knowledge of a professional?