In yesterday's New York Times Magazine, Peter Singer crunched a few numbers and found out that Gatesie-boy et al could actually be pitching in a tiny bit more.

The rich, then, should give. But how much should they give? Gates may have given away nearly $30 billion, but that still leaves him sitting at the top of the Forbes list of the richest Americans, with $53 billion. His 66,000-square-foot high-tech lakeside estate near Seattle is reportedly worth more than $100 million. Property taxes are about $1 million. Among his possessions is the Leicester Codex, the only handwritten book by Leonardo da Vinci still in private hands, for which he paid $30.8 million in 1994. Has Bill Gates done enough? More pointedly, you might ask: if he really believes that all lives have equal value, what is he doing living in such an expensive house and owning a Leonardo Codex? Are there no more lives that could be saved by living more modestly and adding the money thus saved to the amount he has already given?
Yet it was not until, in preparing this article, I calculated how much America’s Top 10 percent of income earners actually make that I fully understood how easy it would be for the world’s rich to eliminate, or virtually eliminate, global poverty. (It has actually become much easier over the last 30 years, as the rich have grown significantly richer.) I found the result astonishing. I double-checked the figures and asked a research assistant to check them as well. But they were right.

(Of course, we're only talking about the super, super rich here, not like the peon's getting a mere $100 million in bonuses, who have much better options for spending their cash).
What Should a Billionaire Give – and What Should You? [NYTimes]