A new thread at SEW explores the concept and value of a link over time.

• Does a long-standing link constitute trust or laziness?
• Are many new links to a site a sign of its  current popularity?
• If those links stay active, how should search enginers consider the site?
• What if some links dissapear?

These questions and others don't have great answers right now and it's something that the web community on both sides of the fence (both IR & SEO) need to look into. One of the major problems will be disambiugating temporal links (those that were put in place and left because no one cleared them out) vs. trust links (those that remain constant because the content creators wish to continue to promote the link).

A secondary source of input and knowledge on this issue comes from an IR paper called - "Trend Detection through Temporal Link Analysis" (warning - it's a PDF). The team working on the paper notes that as of the writing (January 2004), temporal data was not included in major web search engines' analysis of links. They give some strong arguments in favor of collecting and using this data and even suggest search engines monitor traffic levels to sites as a better method for refining temporal information.