My brother Patrick owns a web design firm Modern Blue Web Design and I have always been impressed with not only his design but also his ability to create sites that are 100% valid in whatever the code he is using. He can even validate flash inside of a site, which not too many designers can do.
Here is some seo proof that validation and good overall site structure can help a site be indexed with ease. One of his latest projects is DoGreat.com(awesome domain name). When the site was done he was lookng at some things and found that it had been indexed in Google, but he nor I, nor anyone else had linked to it, which is the best way to get a new site indexed right away. Well anyways now he has over 60 pages indexed in Google and at the time there were no links to the site now there are a few. But it goes to show that if a site is created very well, smooth code, transitional info and with a good structure you can get most of its pages indexed without a sitemap or links.
Also here is a cool site http://www.w3csites.com/ that lists validated sites, there are some awesome designs on there. If you have a validated site get it on there, you will get traffic.