If you put one form of internal navigation in parallel with another you are essentially telling search engines that both paths and both subset pages are of the same significance. Many websites likely lose 20% or more of their potential traffic due to sloppy information architecture that does not consider search engines.

have we been discussing this, hmmm....

sitemaps and footers and bears, oh no

http://www.seobook.com/archives/002021.shtml