I have just put up a post on the Webxopt site showing how to structure 2 and 3 column layouts to makes sure that your main content is always at the top of the HTML page, where search engines like it.
I’m never sure where to post these things these days, but just in case the link is
http://webxopt.com/wordpress/2009/08/web-page-structure-for-great-seo/