By way of Search Marketing Standard's blog post here, I came across the article titled Natural-Born Search Killers by Rob Garner. It points out that as the web matures, web designers and programmers are increasingly faced with more redesigns and fewer designs from scratch. Everyone knows that flash and hidden text blight search engine optimization efforts. Little attention is ever given, however, to the fact that URLs acquire value over time, and the fact that not factoring this into a redesign can mean literally tossing equity into the garbage. And redesigns frequently entail changing URLs. He calls this concept "Link Equity."
He says:
"Changing domains or renaming existing deep URL structures without a solid strategy can wipe out years of gains in links, traffic, sales and conversions, and prior natural search investments."
This is so true, and I have always told people I work with how critical it is to assess the links one has -- especially deep links. When redesigning, one should locate such important links -- and either avoid changing those URLs or redirect each of those URLs to a proper location with similar content. Usually, a 301 redirect is in order in all of these cases.
Even if it cannot be done systematically for every URL, selecting those important links and directing them manually using a few Mod_rewrite rules can substantially mitigate the damages caused by a redesign's alteration to URL-structure.
Ignoring this is literally throwing away equity. Links are the currency in a world where anchor text can even make the president say he's a miserable failure. Maybe only Enron could book it, but it's definately equity in every sense of the word.
I briefly mention this concept in my article in the Fall edition of the Search Marketing Standard, in the context of emphasizing that designing a sound URL-hierarchy is critical to any search engine optimization effort. The blog post mentions me as well. Make sure to read it. I can't wait to see the newest issue -- and not just because I'm in it 
