It's interesting to compare Dave's NY Times keywords with Google trends, the most searched on keywords for the last 24 hours. Almost nothing in common--not that you'd necessarily expect there would be. People looking for the kind of information you get from the Times make up only a small portion of Google users, I'd expect.
Dave's keywords are only updated once every 24 hours, but I expect that that is merely a practical consequence of the 24 hour news cycle of his source. With more sources, who are themselves continuously updating, the keyword river could be as dynamic as you'd want it to be.
Dave mentions that this looks like a leaderboard and indeed it is. Anything that uses popularity (even of keyword usage) to determine relevance is going to end up looking like a leaderboard. I expect that there would be multiple ways of sorting the river and keyword frequency would be just one. Doesn't bother me...
Update: Dave tells me "the keywords are updated every hour, and do a complete refresh every 24 hours on a rolling basis." Cool.