Matt Cutts on search engine freshness:

"I'll Cliff-note it out for you. The authors tracked Google, Yahoo, and MSN over 42 days using 38 German webpages that were updated daily and that included a datestamp somewhere on the page. They measured freshness by looking at each search engine's cached page to see how up-to-date the page was. If you measure success by having a version of a page within 0 or 1 days, Google succeeded a little under 83% of the time, MSN succeeded 48% of the time, and Yahoo succeeded about 42% of the time. "