Obama could conceivably lose the Electoral College while winning the popular vote by the current 3-5 margins shown in the national opinion polls.
He does it by winning core blue states going away (as seems likely), keeping it much closer than usual in deep South states (as also seems likely), while still narrowly losing the critical states of Ohio, Florida and Pennsylvania because McCain lifts more white votes there than Obama gains in black turnout.
What Obama is hoping for is the true 5+ landslide that will almost certainly imply the retained white working class loyalty that flips Ohio and Florida and retains Pennsylvania, which assures victory.
If that happens, the Electoral College will come under a lot of pressure.
But, they'll never get the 3/4'ths of the states needed to amend the constitution.