triggerfinger - 5:27 pm on Sep 24, 2012 (gmt 0)
An SEO that was on the 404 train early after penguin made the good point that it's probably much easier to contact webmasters with good links and get them to update a URL then it is to contact thousands of webmasters of crappier sites and get them to take a link down. Even if its a money page, if Penguin is hurting the money, it may be worth it to drop the old URL (404) and start a new one.
Another problem is with bookmarks/direct to site, which would see a 404 page. You may want to include a nofollow link to the new page so as to avoid a bad user experience/bounced visits.