I recognize that this news from the National Review is a bit of a disaster for political scientists:
As the din of America’s falling headfirst over the fiscal cliff reverberates across the nation, the Obama administration is quietly killing a key economic metric that tells how, and how many, people are voting with their feet. Since 1991 the Internal Revenue Service has been compiling statistics on filers’ addresses, which the agency’s Statistics of Income division uses to show who is moving into and out of every county and state in the nation. As you’d expect, the IRS also knows the aggregate income levels of those who move. So the movements of the most fundamental productive components of the economy — taxpayers — can be analyzed by journalists and economists, or could until now.
The IRS and the U.S. Census Bureau (which provides technical support in reporting tax migration data) have not made an official announcement as to why the program is being discontinued.