The latest issue of Better Software, Jan. 2006, had survey data from 2005 including one slide on certification.
SQE has published some surveys on the subject. The 2005 survey showed 24% certified. In their 2004 survey, only 24.3% were certified by anybody on anything and over 50% found it had no impact on their career. The 2003 survey indicated 22% overall certified and 45% felt it had no impact.