Gamasutra did an awesome article a few years ago talking about the creation of PacMan. Link:
http://www.gamasutra.com/view/feature/3938/the_pacman_dossier.php
It included a fascinating discussion of the Ghost AI behavior. The short version is that the Ghosts can't turn around*, and try for the shortest path to their target tile. The Red ghost is aiming right under you, the Blue ghost is aiming for 3 tiles ahead of you, the Yellow ghost is aiming for 3 tiles behind you, and the Orange ghost is aiming for the center of the map (which he can't reach, so he orbits the spawning area).
This leads to interesting tricks where you manipulate the Blue and Yellow ghost by changing directions right as they pathfind, so that they target a tile that you don't care about and get out of your way until the next fork in the maze.
It is my understanding the current world record holder did NOT memorize map patterns and timing, but by learning the AI behavior and manipulating the ghosts. This was successful because people who memorize the routes are screwed if they mess up timing on a turn or something, but this technique lets you have a fighting chance to recover.
*The ghosts take short breaks every ~15 seconds where they stop targeting you and start targeting an assigned corner of the maze. When a break starts or stops all ghosts suddenly reverse direction as a tell. They reverse direction even if it means not killing you, and even if it means they're going away from their target tile.
