In my view, usability and SEO go hand in hand like HTML and CSS. You can't separate them because both are entirely critical to the successful promotion of a website. Without great usability, you lose conversions, natural inbound links and visitor retention and return rates. Without great SEO, you'll never get visitors to the site in the first place.

A good thread on the subject of world usability day is ongoing at cre8asite and Bill, Kim and the gang have linked to several good resources and links on the subject.