"Calculated annualized growth rates for each county’s share of its state’s population between successive years and 2008, such as 2000-2008, 2001-2008, 2002-2008, and so on, and then averaged the eight rates."
I think that is worked incorrectly, but it's hard to tell. That methodology is fine... It's not the way I would do it, but it might at least be an ok starting point for most places in the country. Fortunately if there is one place in the country that past trends, even recent past trends, don't explain the future it's here. That isn't wishful thinking. There are some basic demographic changes have been behind a lot population trends over the last decade. Trends which can't in and of themselves continue. Most have heard my polemic on these things. Some of this is explained in a reports we did for the county which you can read here via this page: http://alleghenyplaces.com/plan/existingConditions.asp
I also suspect that work really reflects little of the impact of the current recession, although you hope that recession is not a long term steady state. Nonetheless, some of the places that are projected to be growing fastest are having difficult times of late.