FAQ: Can I copyright my game? Are game rules protected by copyright?
"Copyright does not protect the idea for a game, its name or title, or the method or methods for playing it. Nor does copyright protect any idea, system, method, device, or trademark material involved in developing, merchandising, or playing a game."
(United States Copyright Office, "Copyright Registration of Games" [PDF])
According to the U.S. Copyright Office, "the text matter describing the rules of the game" may be protected by copyright "if it contains a sufficient amount of literary or pictorial expression". For example, a passage describing character creation for the Clans of the Sun and Moon, explaining their society and why they tend to have the skills they do, would probably be protected by copyright. A section merely describing the steps involved in rolling a number of dice or expending a number of points on X attributes would probably not be.
The Intellectual Property Section of the Virginia State Bar puts it this way: "To be copyrightable, the material must be original and possess a minimum level of creativity."
As general legal advice, this is solid, but if you want an opinion on a specific work that will hold up in court, you will need to pay an "intellectual property" lawyer for it. If you have money on the line, do not accept the word of anyone but a lawyer who specializes in intellectual property law.
Here are some links you might find helpful in reading more about copyright:
and intellectual property law:
In addition to having the full statute online, the Cornell site has a good search engine and some links to relevant case law.