Michigan had a population decline between July 2006 and 2007 (-30.000) but I expect that Michigan's population will only have declined slightly between July 2007 and 2008 or even stagnated.
Who would want to stay in Michigan, in this economy?!?!
It might be that people in Michigan feel that opportunities aren't much better elsewhere, that the whole country's in the economic crapper, so why move from one location in the toilet bowl to another.
Or that we value things like quality of life and such.
Of course ancestral and family ties to a region can always explain any decision to not move from any place, also.