"Don't be a dick."
Wheaton's Law is a silly way to refer to a good guideline for healthy social interaction, as it has been espoused by nerd-at-large Wil Wheaton. The general sentiment has been around for years before it was enshrined under this particular name, so a little background is worthwhile to explain why this variant has caught on.
For those unfamiliar with Wil Wheaton, he's an actor and writer, best known by the public at large for his roles as Gordie in Stand By Me and Wesley Crusher in Star Trek: The Next Generation. But those roles were years ago. And the secret all we geeks didn't realize while watching him grow up on screen? He's a geek too and always has been. In good part through his popular personal blog, WWdN: In Exile (Wil Wheaton dot Net: In Exile), he's become one of those weird Internet-age celebrities. He has thousands of regular readers of his blog, but still responds personally to comments, and he displays his passions and feelings with very limited filtering. He seems inches away, and that pseudo-intimacy with someone you can identify with and admire is intoxicating.
So in 2007, Wil Wheaton stood as a figure of respect in the community of geeks at large and gamers in particular. And he was asked to be the keynote speaker for the approximately 39,000 attendees of that year's Penny Arcade Expo (PAX) . To place this in context, 2005 and 2006 saw several press conferences condemning video games, overseen by the grandstanding of individuals such as Hillary Clinton and Jack Thompson. The recent release of Grand Theft Auto: Vice City and the leaking of the "hot coffee" mod were getting negative press coverage. By and large, "the dangers of violent video games" were in the news in a bad way. Those of us who played games and didn't go around committing murder afterward felt a little like our hobby was being unfairly slighted.
And so the PAX keynote of that year concentrated on the redeeming values of videogames, and, in particular, the video arcade scene of Wil Wheaton's youth:
"...they were a vital part of my generation's social upbringing. If I beat another kid in a two player game and taunted him mercilessly with explicit references to his mother's sex life and my role in it, the way some online gamers do today, he would have justifiably kicked the ever-living-shit out of me. So I learned, in arcades, the importance of good sportsmanship.
"Because arcades were physical places, staffed by real people. We had to worry about a lot more than getting kicked off a server if we were complete idiots in a game. And I feel like a cranky old man by bringing this up at all, but there's a lot of you here. So, would you do me a favor? When you're playing online, have fun. But don't be a dick, okay?"
The crowd cheers.
"I can't tell you how good it makes me feel to know that there are so many people here who agree with that. Maybe I'll go ahead and sign up for XBox Live after all." 
The keynote was not just well-received in the expo hall, but was widely reported and embraced by gaming media covering the event. Since then, Wil Wheaton has been a regular attendee of PAX, holding a panel to meet fans at all subsequent PAX events and giving the keynote at PAX East in 2010. His lesson in good sportsmanship has been further enshrined in a song by nerdcore artist MC Frontalot, "Your Friend Wil" 
Asserting Wheaton's Law is sometimes an effective away of defusing a situation, depending on the level of agitation already present. Humorously alluding to the current tenseness, depending on the quality of the individuals involved, may allow everyone to back off and loosen up. But, let's face it, the easiest way to get someone to be more of a dick is to tell them "Don't be a dick," even if you're being indirect about it. So if Wheaton's Law comes into play after the fact, it may already be too late.
But maybe, just maybe, if we can all take Wheaton's Law into our hearts, some day we can play Settlers of Catan without losing any friends.
Primarily for your listening pleasure, but feel free to fact check me while you're at it.
http://wilwheaton.typepad.com/wwdnbackup/2007/08/pax-ftw.html, "PAX FTW", Wil Wheaton, "WWdN In Exile", 2007 August 27
http://www.pennyarcadeexpo.com/PAX07_Keynote.mp3 (starting at minute 21), Penny Arcade Expo 2007 Keynote Speech, Wil Wheaton, 2007 August 24
http://frontalot.com/index.php/?page=lyrics&lyricid=68, "Your Friend Wil" (lyrics and MP3), MC Frontalot