I don't know if anyone has seen this, but this site has some great population data for cities all over the world, going back to the 18th Century.

In the US, you can find the population of most towns and cities all the way from 1810 to 2000 all on one chart. Click on a state yourself to see.

http://www.library.uu.nl/wesp/populstat/populframe.html

I just found out that Toledo is now bigger than Buffalo!