From Human Transit, where I am increasingly impressed by the frequency and quality of the posts.
Can we make density make sense? is the latest post about, well, how to handle density data.
Main conclusion is that density per population--What percentage of the population lives at densities above x? What's the average density at which people in this city live?--may be more useful than density per area--people per square mile, or whathaveyou.
Both this new post and its predecessor have excellent comment threads, for those with greater than average curiosity.