After doing my gigantic plot, I found out that there was actually newer census data than the 2000 census data I was using.
That data comes from the American Community Survey (ACS), which is done pretty much continuously, and generates new estimates of key demographics also pretty much continuously. Some of these 2008 numbers are different than the 2000 numbers!
However, having made this mistake, I will justify on it on several grounds: first, the census data, while older, is more complete. The census really does try to count every single person in the country --- while the ACS just estimates it. Also, the ACS doesn't seem to try to take a picture of every single county, which is one of the purposes of my plots.
Although, I am certainly waiting for the 2010 census data to come out.