There has been some discussion on further breaking it down, so I just posted this in the 'Midwest' group.
OK, making progress. I found http://www.census.gov/geo/www/us_regdiv.pdf
which gives the 'official' regional breakdowns as recognized by the U.S. Census Bureau. However, looking it over shows there's no 'Northwest' or 'Southwest'. Also the 'West: Mountain' region is huge. But, it breaks down the USA into 9 regions. After looking at the map it makes some sense, and I might as well go with what the census uses.
Northeast: New England (CT, MN, MA, NH, RI, VT)
Northeast: Middle Atlantic (NJ, NY, PA)
Midwest: East North Central (IN, IL, MI, OH, WI)
Midwest: West North Central (IO, KS, MN, MO, NE, ND, SD)
South: South Atlantic (DE, DC, FL, GA, MD, NC, SC, VA, WV)
South: East South Central (AL, KY, MS, TN)
South: West South Central (AR, LA, OK, TX)
West: Pacific (AK, HI, OR, WA)
West: Mountain (AZ, CO, ID, NM, MO, UT, NV, WY)