Using the webmaster tools, I've seen not found errors show up in the crawler stats for pages that were 3 years old (and three years gone). They appear a half-dozen or so at a time, and then are replaced by a new set of old pages. One of my sites is an event calendar, and the daily calendar url's expire, well, daily, yet googlebot is still looking for them three years later. I have a sitemap.xml file that drops the old urls daily, yet they still have ancient urls tucked away somewhere.
I get the sense that Google is so distracted with all their 20% projects and gizmo's that they are not keeping up with search. I mean, getting sued by Viacom for a billion bucks has got to be pretty distracting, I presume.
Also, based on the original post, has anyone else seen their site dropped hundreds of placements in G due to old broken links on their site? Given the economic impact of placement, it's kind of sickening if that's true, and it's not the webmaster's fault.