In his own words [makes ya wanna cry, don't it?]:
"To whom it may concern,
Many years ago, I put virtually all of my assets into a trust with the intent of giving away at least 95% of my wealth to charitable causes. I have already given hundreds of millions of dollars to medical research and education, and I will give billions more over time. Until now, I have done this giving quietly – because I have long believed that charitable giving is a personal and private matter. So why am I going public now? Warren Buffett personally asked me to write this letter because he said I would be “setting an example” and “influencing others” to give. I hope he’s right.
From The Giving Pledge website: http://givingpledge.org/#larry_ellison