Here's an interactive tool at he the NY Times that looks at 512 different scenarios for wins by either Obama or Romney depending on who wins in the 9 battle ground states of Florida, Ohio, North Carolina, Virginia, Wisconsin, Colorado, Iowa, Nevada and New Hampshire.
Of the 512 paths to victory for either candidate, Obama wins in 431 (84.2%) scenarios. Romney wins in 76 (14.8%) scenarios. And there's a tie in 5 (1%) scenarios. It's a handy tool to use during the returns on Tuesday night.
As you can see from the various scenarios, Romney would have to have an extraordinary run to make it to the White House at this point.