Bill Gates and Warren Buffett's call this week asking the world's billionaires to give away half their fortunes has been met by a touch of skepticism by some who wonder if the motivation is one of true charity.

Gates and Buffett announced this new initiative, called the Giving Pledge, which calls on everyone on the Forbes 400 American rich list to pledge half their net worth -- either now or at their death. The hope is that if everyone on the list agrees, the plan could raise US$600 billion.

Buffett has already written his pledge, which is published on the Giving Pledge website --though he's going beyond the 50 per cent target, aiming to give away 99 per cent of his $50 billion fortune.

Although the letters will not be legally binding, they are intended to create a sort of "moral obligation." But by signing on, donors will also be invited to an annual "Great Givers" summit with Gates, Buffett, and more high-rollers.

So far, a handful of the world's mega-rich have signed on, including:

• Los Angeles philanthropists Eli and Edythe Broad
• Silicon Valley's John and Tashia Morgridge, whose fortune came from Cisco Systems
• venture capitalist John Doerr of Kleiner Perkins and his wife, Ann
• media entrepreneur Gerry Lenfest and his wife, Marguerite

Buffett along with Bill and Melinda Gates are sending e-mails and making calls to other billionaires deemed likely prospects to contribute, reports Fortune magazine.

Matthew Miller, a research officer with Wealth-X, a company that tracks the spending of the world's wealthiest people, says Gates and Buffett likely have more on their mind than pure philanthropy.

"Bill Gates and Warren Buffett are a funny bunch in that they're looking at their legacy," Miller told CTV News Channel earlier this week.

He notes that Gates is mostly retired from Microsoft and is now focused full-time on philanthropy. Buffett is 80 and nearing the end of his life as one of the finest investors the world has seen. He says neither man is interested in quiet charity.

"Remember that both of these people, as No. 2 and No. 3 richest people in the world, are both egomaniacs. So while there's a lot of good going on in this philanthropy, there's also a lot attention that they're going to get," says Miller.

Others who step up to the challenge may also be making the move with their public image in mind, suggests Miller. At a time when much of the world despises Wall Street for helping spur the last recession, wealthy businesspeople are having a bit of a PR crisis at the moment.

"The billionaires who care a lot about their image will step up to the challenge," predicts Miller. "They'll see this as an opportunity to associate themselves with Bill Gates and Warren Buffett, two of the greatest philanthropists in the history of America – or at least that's what they're trying to be."

The other player likely motivating Gates, Buffett and other billionaires? The taxman.

Wealthy people who don't move most of their fortunes out of their names by the time they die risk seeing large portions of their fortunes clawed away by the government in estate taxes.

"But there's a flip side to that," notes Miller. "Most of the richest people in the world have their fortunes tied up in private companies or large blocks of stock. You can't just give that away. If you gave away half a private company, you'd have to fire thousands of employees," he says. "So asking people to give away half their money… is easier said than done."

John Tammy, an economist who writes a weekly column for Forbes.com, says he doesn't think billionaires should give away their money to charity.

Tammy believes that doing so will only help people in the short term, whereas keeping it invested in business will help people long-term. He suggests a learning and business-focussed type of charity.

"The removal of limited capital from the productive parts of the economy will ultimately reduce our standard of living, drive up unemployment and make individuals more -- as opposed to less -- needful of charity. Conversely, money saved and invested constitutes capital offered to today's and tomorrow's businesses," he writes.

For those who do choose to give, the pledge doesn't obligate them to donate to specific causes – only that they donate at all.

Nathaniel Whittemore, the editor of Social Entrepreneurship at Change.org says he hopes that billionaires choose charities that address "high impact problems" affecting the world's poorest.

"The biggest thing that worries me about this is that the institutions that have traditionally been the refuge of rich philanthropic dollars -- universities, symphonies, etc. -- are all well and good but only a tiny, fractional slice of what we need resources for," he writes.

The idea of challenging people to donate a portion of their fortunes is not a new one of course. Many of the world's great religions have obligated followers to tithe 10 per cent of their earnings for millennia.

Other more grassroots groups have made more modest calls. For example, Giv3 encourages Canadians to donate three per cent of their income every year to charity, while also volunteering three hours a month.

While that doesn't sound like much, the group notes that most Canadians give less than once per cent of personal pre-tax salaries to charity, with lower income groups giving a higher percentage of their income (1.7 per cent) than higher income groups (just 0.5 per cent).