I'm one of the founders of Code For The People - we're on the VIP Service Partners list, and yes, we absolutely consider that to be our formal certification of competence, for those who care about such things. Most of our enterprise-level sales inquiries come purely because we appear on that list.

But when it comes to finding freelance (or permanent) staff, we use the Mika method. Your participation on trac, your inclusion on the Credits list, your list of themes/plugins in the official repo or Github, your Twitter account even - these things tell us more about you than your résumé or any formal certificate. That's the beauty of open source.

I've worked in parts of the IT industry where certification was considered essential. There's a reason I switched to working with WordPress.