I found the ruling to be an interesting read which went to lengths to distinguish idea from expression. It even referenced an older case of Atari vs. Amusement World (Asteroids vs. Meteors) which Atari lost:

http://scholar.google.com/scholar_case?case=6441518363892064...

A notable quote from that case:

  It seems clear that defendants based their game on plaintiff's 
  copyrighted game; to put it bluntly, defendants took plaintiff's 
  idea. However, the copyright laws do not prohibit this. Copyright 
  protection is available only for expression of ideas, not for ideas 
  themselves. Defendants used plaintiff's idea and those portions of 
  plaintiff's expression that were inextricably linked to that idea. 
  The remainder of defendants' expression is different from plaintiff's 
  expression. Therefore, the Court finds that defendants' "Meteors" 
  game is not substantially similar to and is not an infringing 
  copy of plaintiff's "Asteroids" game.

The Munchkin ruling also references an older case and quotes:

  Thus, "if the only similarity between plaintiff's and defendant's 
  works is that of the abstract idea, there is an absence of substantial 
  similarity and hence no infringement results."

It later goes on to say:

  Plaintiffs' audiovisual work is primarily an unprotectible game, but 
  unlike the bee pin, to at least a limited extent the particular
  form in which it is expressed (shapes, sizes, colors, sequences,
  arrangements, and sounds) provides something "new or additional 
  over the idea."

This was to be North American's downfall:

  North American not only adopted the same basic characters but also 
  portrayed them in a manner which made K. C. Munchkin appear 
  substantially similar to PAC-MAN. The K. C. Munchkin gobbler 
  has several blatantly similar features, including the relative 
  size and shape of the "body," the V-shaped "mouth," its distinctive 
  gobbling action (with appropriate sounds), and especially the way in 
  which it disappears upon being captured. An examination of the K. C. 
  Munchkin ghost monsters reveals even more significant visual 
  similarities. In size, shape, and manner of movement, they are virtually 
  identical to their PAC-MAN counterparts. K. C. Munchkin's monsters, 
  for example, exhibit the same peculiar "eye" and "leg" movement. 
  Both games, moreover, express the role reversal and "regeneration" 
  process with such great similarity that an ordinary observer 
  could conclude only that North American copied plaintiffs' PAC-MAN.

Had they only copied the idea the case would have gone the way of the Asteroids vs. Meteors case. Where they went wrong is that they copied the expressions used to such a degree that it was found to be infringing their copyright.