States losing seats include New York, New Jersey, Massachusetts, Pennsylvania, Illinois, Iowa, Missouri, Michigan, Ohio and Louisiana.
Here's the full breakdown, via the US Census Bureau:
The 2010 Census numbers are actually early; they're due to President Obama on Dec. 31, 2010. Obama will then present the data to the 112th Congress when it convenes in January. During 2011, states will work to redraw the lines of their congressional districts.
States gaining or losing seats will have to fit their populations into a new number of districts. For states like New Mexico, which retains the same number of seats, redistricting will focus on accommodating the state's population growth. The newly apportioned and districted Congress will convene in 2013. Check out the population growth percentages driving reapportionment:
Here's a cool graphic for how reapportionment has shifted political weight among states in the past:
What does it all mean? This year's census involves a shift in the country's population center away from the Northeast and Midwest and toward the southern and western states. According to Bloomberg, much of that growth is due to Hispanic voters--which could mean that red-state gains don't necessarily translate into additional seats for Republicans. (Huge caveat: immigration reform.)
Regardless, much depends on how the districts are drawn--an often heated and ugly battle for seats sometimes given to ending up in court.
But the breakdown is stark:
GOP presidential vote, 2008: Texas ( 4), Arizona ( 1), Utah ( 1), Georgia ( 1), South Carolina ( 1), Louisiana (-1), Missouri (-1)
Dem presidential vote, 2008: Florida ( 2), Nevada ( 1), Washington ( 1), New Jersey (-1), Massachusetts (-1), Pennsylvania (-1), Illinois (-1), Iowa (-1), Michigan (-1), New York (-2), Ohio (-2)
GOP net: 6
Dem net: -6
So, yeah. We'll see.