From Stephen Von Worley at Data Pointed (via Flowing Data), maps of block-by-block population changes from 2000 and 2010 for a bunch of American cities. They show that growth continues to occur mostly at the fringe of cities, which is hardly surprising. The changes in central city populations are more interesting because you can tell at a glance which central cities are healthy and which are not.
"[D]eep blue indicates that the population doubled (or more), pure red means that everyone left, grey denotes no change, and the intermediate tones represent the spectrum of increases and decreases in-between."
Here is Austin:
You can compare Ryan Robinson's map of the population changes by census tract.
It would be wise to exercise a little caution with these maps. Census blocks frequently change from one census to the next, which makes comparisons tricky. I'm not sure how the author addressed this problem, if he addressed it at all. [Update: Steve emailed me to say that he "used the block data for each Census to compute the gridded population for that year, then took the difference. Nice work!]
Here is what a 25% loss in population (Detroit) looks like: