According to a state-by-state analysis conducted by Matt McDonald, a partner at the GOP-aligned Hamilton Place Strategies, the unemployment rate outpaced the national average in only four swing states last month: Florida, Michigan, Nevada and North Carolina. ... Those four states will account for 66 electoral votes in 2012. Both parties are likely to target 10 states that have unemployment rates below the national average Colorado, Iowa, Indiana, Minnesota, New Hampshire, New Mexico, Ohio, Pennsylvania, Virginia and Wisconsin. Those states have a total of 106 electoral votes.
As Chris writes, that means Obama could lose all four states where unemployment is above the national average and assuming he can retain the other states he won in 2008 still win a relatively comfortable reelection with 299 electoral votes.
http://www.washingtonpost.com/blogs/ezra-klein/post/swing-state-unemployment/2011/05/19/AGDF88cH_blog.html