I don't think a certification could hurt, but as far as design goes, I would not think it would benefit you as much as a strong portfolio. When I was a network administrator, having Microsoft Certification would be the deal-maker for a job. With regard to design I think your work speaks for itself.
There are plenty of awful sites that proudly showcase their memberships or certifications from groups like the html writers guild or the various "awards" their sites have recieved. You wonder what credentials those who give out such certificates have or if they even viewed these sites.
If you can learn something from the people providing the certifications that you can't get elsewhere and there is a respected community of designers holding such certifications it may be worth it. Potential employers / clients will make decisions on how much they like your work, you and how much you charge.
The CIW is run by a university in glasgow also; i was due to enrol but to be honest it wouldnt teach me anything i hadnt picked up in the past few years alone.
What finally put me off; i searched the CW jobs index (http://www.cwjobs.co.uk it + design based uk index) for a few months, but never any mention of CIW certification required.
People get mixed up with the titles; if its all in web design pakages you want to offer then enroll in an HND / equivilant at your local college. You'll learn to intermediate level the in's and outs of the 'webdesigner' packages like photoshop and dreamweaver.
High quality design houses have Graphic Designers who are qualified in Graphics, and Coders who are qualified in computing / programming etc; hence you should pick one or the other if you want to do it full time in the city etc.
All in i reckon some big names in your quality portfolio speaks a lot louder than any certification.
I decided to go for the HND Internetworking which starts in august, perhaps along the second / third year i'll make a start on the cisco or MS certification.