Invalid markup can cause the document tree to have misplaced elements and the search engine won't be able to follow the tree properly. Just like browsers can't follow the model either.
Validation is not focused so much on SEO i feel, apart from the point that drhowarddrfine made. Basically you're not going to score bonus points for everything validating (SEO wise) but obviously it cant be doing any harm to actually have pages that do validate.
Validation is not focused so much on SEO I agree .... thaT method does not matter ...... it would be more about tagging and contents on each pages here look at this page based on the uri and categorized uri info it reflects on seo not validation ... every time you validate it comes out wrong any way ... look at this uri for example Kauaimassages.com/Kauai-Guide/Kauai-Golf/index.html note how deep it gets at the end ... bringing visitors into that category they look for and surfing that contents... ... Learn how to use Dream Weaver and your more likened to find out how everything in webdesign and uri structure makes sense...... DW is the besteditor ... people are finding that out but learning it calls for not only a master but a person that knows SEO
ive always thought that a correctly coded site will give u a better ranking but i may be wrong... but generally having a well coded site is good anyway as the above ppl have said