My sense is that if the linking page goes stale, then the links get devalued. I even think there may be periodic sweeps through the entire webgraph to downgrade links from very old and stale pages.
Ted, this is why I continually backlink to existing pages with each new page of content that is added. It is impossible, of course, to cover all pages as you create more content, but I've viewed it as a way of telling google "here it is, don't forget about this one, it's still important too". I think it helps to keep the flow of pagerank a fresh and clean stream versus something that's stale and muddied. And the constant inclusion of backlinks on new pages hammers home the relevance factor for the linking page and the linking target.