usability factor into SEO?

Search engines don't have access to the usability data of every website. Though, as more people install Google Analytics or Adwords tracking code onto their websites, and surfers install the Google Toolbar into their browser, Google is able to get more and more of this information and can incorporate it into their ranking algorithms (though they claim they don't and won't.)

Short of using that data, search engines can see visitor bounce rates pretty well. Every visitor that clicks over to a site, then hits the browsers back button, sends a signal to the search engine that the visitor didn't find what they were looking for.

Bounce rates can certainly be an important signal to the search engines. Making sure your visitors get the information they searched for, on the first, click is imperative.

Given access to enough data, I think the engines will look more closely at on-site usability issues. If few people make it to the conversion page, the engines may determine that your site isn't a quality or trusted site, especially if it is compared to another site that does have better conversion rates.