Bill Gates and Warren Buffett have been leaders by example to others of extreme wealth since they committed major parts of their fortune and energy to changing the world while they were still alive. With the just-launched Giving Pledge, they're making that leadership explicit and asking other billionaires to commit at least half of their wealth to philanthropic causes.

Bill & Melinda Gates have been philanthropic leaders for years. Their foundation was first formed in 1994, but has grown into the largest private foundation in the world largely in the last half decade or so. The turning point event was a gift by Warren Buffett worth approximately $30 billion in June 2006. The gift coincided with Bill Gates shift from his role at Microsoft to full-time leadership of the foundation.

The immediate origins of the Giving Pledge came at a clandestine billionaire meeting a few months ago, at which the Gates and Buffett convened some of the more philanthropically active wealth in the country: David Rockefeller played host and the event included people like Ted Turner, Eli and Edyth Broad, and yes, Oprah. The conversation was ultimately focused on how to increase giving among the rich, and while many ideas emerged, the only consensus was that it would take lots of engagement and lots of work.

Over the course of the next few months, meetings were held in secret around the world getting the ultra-wealthy -- the targets of this cohort were and are, at first, the Fortune 400 -- to increase their giving. At these meetings, the idea of pledging some percentage of net worth was floated and not shot down.

The Giving Pledge has set the bar at asking people to pledge to give away 50% of their net worth to philanthropic causes. The Gates and Buffett seem to think that number should be even higher, but made the decision to set it at a level that they thought they could pull people in with.