Squarespace sites come completely optimized for SEO out of the box without you having to do anything at all. A quote from the CEO of SEOmoz, the web's most respected SEO community:
The Squarespace team has put together a remarkable platform. Every aspect of the product - user experience, featureset, SEO friendliness, design and more - all work together to create one of the few content management systems I'm proud to recommend.
(You will also notice that Squarespace is in the screenshot on the front page of SEOmoz)
Further, as a technical summary:
Squarespace produces pages wit