Fivethirtyeight.com, probably the most reliable of the poll interpreters, lays out 4 possible ways McCain can win the electoral votes. However, a popular vote win seems unlikely.
The first map is actually within grasp for McCain. Missouri, North Dakota, and Indiana seem to be trending toward McCain. North Carolin and Florida are virtual ties. If McCain pulls out a win in Florida, which seems more likely now than a win in North Carolina (despite how strange that sounds), that means the election rests on 4 states. Virginia, Colorado, Ohio and Nevada.
McCain has bet pretty much everything on pulling out a win in PA. However, in PA the last five polls weighted by accuracy and reliability by 538, all favor Obama by an avg of about 6 points. But in Ohio, the last five most reliable polls has McCain winning two of them, and Obama avg only a 4 point lead in the rest.
In fact, McCain can win without Pennsylvania if he wins Ohio, Colorado, Virginia, and Nevada and pulls an upset in New Hampshire. The reason they are betting on Pennsylvania is that if he wins Ohio, but loses Penn,, yet holds onto Virginia, Colorado, and Nevada, its the 269/269 tie, which would then go to a house vote, meaning a President Obama.
If the two candidates split Ohio and Pennsylvania, that leaves us with Virginia, Colorado and Nevada. Virginia seems pretty safe for Obama, but is definitely close. If he wins VA, the election, in any McCain win scenario, comes down to the two interior west states: Nevada and Colorado. McCain needs both. Nevada is a tie right now. Colorado has a slim lead for Obama, just outside the margin of error. If Obama wins Colorado there is essentially no way for McCain to get past 269 unless Obama also loses both PA and Ohio, a scenario very unlikely the happen.
Here are key states which, early on, will be good gauges to see how the night is going:
New Hampshire: If it goes McCain, Obama may be in serious trouble.
Pennsylvania and Ohio: If Obama wins both, he's the next president. There's no way for McCain to win. If McCain wins either it's still anyone's election. If McCain wins both he can afford to lose a state or two, and Obama has to win either Florida, or Virginia, Colorado and Nevada.
North Carolina: An Obama win here would be a sign of a great night for the Senator from Illinois.
And if you're still not sold on the idea that the Electoral College needs either scrapping or severe reform, a system's analyst has run a model where a candidate could only win 22% of the popular vote and still be elected president. See how here.