Is a formal Certification in Testing still Relevant?

The real answer to this is - it depends! on the context, on the need, on necessity, on motivation, on relevance, and even on geography...
Is it absolutely necessary - definitely not! Should you go for it - may be. Why? - read the first line.

OK, now the real reasons.
What a formal Certification (like ISTQB) does is a lay a foundation about the basic process of Software Testing, that anyone who works in this field is expected to know - you cannot build an Empire state building with only 6 inches of slab as foundation.
But I feel that the course-ware (syllabus) is mostly irrelevant in today's real world scenario, though some of it can be pretty useful even in the advanced stages of your careers. It is like the basics of engineering that we all studied but rarely used, but it was still important at that point of our lives.
So, the point is if you are just starting your career in testing - most of the young people in India and some Asian countries fall in this category  - it might be worthwhile to go for it. Because most testers start their careers in testing without any formal training in the different aspects of testing, and they keep struggling until they develop that knowledge and understanding. So all this certification can do is speed-up that learning a bit, by pointing you in the right direction.

But for people who have credible experience it may not be worth spending a weekend that you could have spent on taking your wife shopping! For people in mature markets like US and EU, this certification has already lost all relevance; and for those who want to 'statistically' refuse, here is a good article that can help you do just that - http://chrismcmahonsblog.blogspot.in/2010/10/ignoring-certification-with-numbers.html

A weekend is all it takes to clear these certifications - but its no cake-walk either as the failure rate is quite high, though the material is quite abundantly (and freely) available online.

One exam tip for those who want to take the plunge - don't sweat too much over the 'Standards' even if you get the itch to do so, because not much gets asked about those anyways!