TrueHoop has an interesting look at the Paul Pierce vs. LeBron James rivalry.
That "have a history" link Windhorst included is to a 2006 Windhorst piece making a pretty convincing case that as much as rivalries with Dwyane Wade, Carmelo Anthony, or Kobe Bryant may be hyped, Pierce is James' truest rival in the NBA.
At the time of Windhorst's article they had faced each other more than any other stars. They had all kinds of big scoring games against each other. Unlike a lot of more hyped rivals for James, these two have actually spent a lot of time guarding each other.