Just a quick observation...

A couple of months ago I tackled a site with 100's of W3C compliance errors. It was a long and boring process.

I've just been reviewing the campaign and the Google rankings are fascinating viewing. They shot up a couple of days after I uploaded the newly validated page and have stayed consistently high ever since. No other changes happened at the same time.

The importance of W3C validation in SEO is a bit blurred, but having seen this remarkable improvement I am pretty much convinced that it is worthwhile.