In 2008 McCain would have had to win the popular vote 2.5% to get to 270 EV. So I expect this year Obama could win the electoral vote and lose the popular vote by up to 2%
I wanted to demonstrate this visually, because I think this may say something about 2012's EC landscape.
If McCain had won by 2.5% (assuming third party totals remain unchanged), that would make the margin 50.49 to 47.99 in favor of McCain. That's a 4.88% swing from Obama to McCain. Map:
McCain 280, Obama 258
The lowest possible McCain EC win comes with a 2.39% victory in the PV that delivers Iowa and Colorado (with a 2.24%, McCain picks up just CO and ties 269-269*). It's interesting to note that 2.38% of the vote in 2008 was ~ 3,312,476 ballots. McCain could have won the PV by over 3.3 million votes and still lost the election (hypothetically).
[size=8.5pt]*Romney, after the redistricting, can actually win with a 2.24% margin based on 2008 numbers. That map equals to 275-263.[/size]
The winner take all EC system is supremely flawed in this regard. The President is elected to lead the entire nation, coast to coast, and should be elected as such as opposed to the EC system that allows for divisive regional strategies. We need a popular vote system.