freejung - 11:32 pm on Feb 16, 2011 (gmt 0)
The problem with talking about validation as an "SEO technique" is that, on the one hand, certain kinds of coding mistakes can prevent you from being indexed properly at all or prevent juice from flowing properly through your site. If that's what's holding you back, then fixing those errors would be the #1 priority and do great things for your SEO.
If that's not what's holding you back (if your indexing and juice flow are fine) then fixing obscure validation errors like having more than one element with the same id is probably not going to help much.
So it's a great tactic _if_ you have major problems with it, but not otherwise.
With the other approaches discussed (info architecture, link building etc.), there is almost always room for improvement.