Only for Adsense purposes, not for organic search. However, Google and Bing are both becoming quite sophisticated about digging out the important stuff.
In a pinch, you can publish the insignificant text on a separate URL and just iframe it wherever you want users to see it, but not have search engines apply any weight to the page where the iframe appears.
That's exactly (one of the things) XHTML was designed to do. You write a custom DDT to define those "ignore me" tags.