I thought this might be a good place to drop this article. It reviews several past copyright cases for video games - going as far back as Asteroids! The article also very clearly explains that:
Copyright law does not protect against the borrowing of the underlying idea in a creative work - only the specific expression of that idea. The rules, game mechanics, and functionality of a video game are said to "merge" with the underlying idea. This means that game mechanics and the rules are not entitled to protection. Only the "expressive elements" of the game are copyrightable.
Specific decisions on each ruling are discussed, and may help readers to better understand how copyright law is applied.