The very useful site 270ToWin.com lets you play out the 2012 elections with various re-apportionment scenarios. The first thing I notice is that if reapportionment goes as most people expect it to -- if rust belt and New England states lose seats, while the Sun Belt gains them -- and Barack Obama won exactly the same states that he won in 2008, he would win 359 electoral votes. That's down from the 365 votes he won in 2008. (Both numbers assume that Obama can win the extra electoral vote he won by taking Nebraska's 2nd district, which centers around Omaha.)
What does this mean if the 2012 election is tighter? Well, let's assume that the trends of 2008 and 2010 continue, and Obama loses ground in the South and the Midwest, but stays strong in the Southwest. If he won the states Al Gore and John Kerry both won, he is at 243 EVs. If he adds Colorado, New Mexico and Nevada to that total, he will be at 263. From there he can claw back Iowa and New Hampshire for 273 votes, win Virginia for 276 votes, or find some other path. By comparison, if the states were apportioned as they were in 2008, he'd only need to win Iowa OR New Hampshire.