North Carolina is very close to joining Maine and Nebraska as apportion states in the Electoral College. Arkansas and Louisiana are also exploring the idea. How this works is as follows:
The winner of the overall popular vote in a state gets the two at large electoral votes. Then, the winner in each congressional district gets a vote.
As an Arkansan, I can tell you that if this had been in effect even only in this small state, Al Gore would have been President. Gore carried the 4th (Pine Bluff) and 1st (Jonesboro - Helena). He would have gotten two votes from Arkansas, and Bush would have gotten four. That would have made his total 271, or one more needed.
All three of the states exploring the change are basically red states with blue districts. This means more electoral votes for Democrats every time.
In the past, this hasn't come into play, even in the states that apportion. Maine is so blue and Nebraska is so red that the Democrat has carried both of Maine's CDs in recent elections and the GOP has carried each of Nebraska's three districts. (The Omaha - Lincoln district is competitive.)
MyDD with details.
To illustrate the impact, explore Dave Leip's Political Atlas, note the CD split in each state. (Important note: For this site, Democrats are RED and GOP is BLUE.)
Exact figures: Would have given Kerry 4 more votes in '04 and Gore 6 more in 2000 if change had been in effect in all three states.