Billionaires are holding mountains of cash, offering the latest sign that the ultra-wealthy are nervous about putting more money into today's markets. According to the new Billionaire Census from Wealth-X and UBS, the world's billionaires are holding an average of $600 million in cash each—greater than the gross domestic product of Dominica. That marks a jump of $60 million from a year ago and translates into billionaires' holding an average of 19 percent of their net worth in cash. "This increased liquidity signals that many billionaires are keeping their money on the sidelines and waiting for the optimal moment to make further investments," the study said.

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Indeed, billionaires' cash holdings far exceed their investments in real estate. Their real-estate holdings average $160 million per billionaire, or about one-fifth of their cash holdings. Simon Smiles, chief investment officer for Ultra High Net Worth at UBS Wealth Management, said that the billionaire families and family offices he talks to are focused largely on the same question: What to do with all their cash.

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"The apparent safety of cash, reinforced by the painful psychological experience of the 2008-09 global financial crisis and the subsequent troubles within the European Monetary Union, likely reinforces the tendency to favor this cautious allocation strategy," Smiles said in the report. But he said creeping inflation threatens to erode cash values, so he's advising clients to take on "considered amounts of risk" with interest rate swaps, credit default swaps, or selling rates or foreign exchange derivatives. Read MoreChinese millionaires plan to leave in droves: Poll

Yet in today's increasingly frothy market environment, and after the hangover of 2009, today's billionaires prefer a return of their assets rather than a return on assets. And in fact, they may be happy with a small loss rather than risk a larger one.