Most people in the 60,000-strong crowd barely noticed the stunt on a rain-lashed night, but the world's media latched on to the protest and gave it prominent mention in their reports of the eagerly awaited performance by one of rock and roll's biggest acts.

Years ago the band transferred some of its assets from Ireland to the Netherlands, as did members of the Rolling Stones, prompting the New York Times to label the Netherlands "The New Tax Shelter Hot Spot."

U2's music triumphed on that muddy June night, but the tax habits of the world's super-rich have since become a hot topic.

Cash-strapped western governments are on the offensive against their own elites. Wealth taxes are up, loopholes are being closed and crackdowns on offshore havens and Swiss bank accounts are gaining momentum.

"Flogging the rich always becomes a national sport in times of crisis," says Catherine Tillotson, managing partner at consultancy Scorpio Partnership.

Some super-rich are volunteering funds: billionaires Warren Buffett and L'Oreal SA heiress Liliane Bettencourt recently made statements offering to shoulder more of the tax burden.

In the United States, President Barack Obama has called for a new minimum tax called the "Buffett Rule" for American households that make more than $1 million annually. A recent USA Today/Gallup poll showed 66 percent of Americans support increasing income taxes for wealthy individuals.

Proposed changes to the U.S. tax code may not materialize soon — the proposal is a "political statement and not a serious legislative proposal," said Scott A. Hodge, president of Washington-based research group the Tax Foundation.

But the debate and recent moves have underlined a longstanding truth: it is much harder for wealthy Americans to avoid taxes than for their European counterparts.

"You are definitely better off not being American," said John Christensen, director at Tax Justice Network, which campaigns against tax havens.

NO ESCAPE

U.S. citizens are liable to U.S. tax wherever they are in the world, making it virtually impossible for them to become legal tax exiles — a possibility open to Europeans, many of whom have set up home in tax havens like Monaco and Britain's Channel Islands.

Americans cannot even escape these obligations by renouncing their citizenship, says Sydney E. Unger, a partner in the tax department of New York law firm Kaye Scholer LLP.

"If you want to renounce...you have to file tax returns for (the) last five years, and there is now an imposed exit tax. If you have a lot of assets, you have to pay tax on them as if you had sold them," he said.

Since the 2008 financial crisis, tax advocates say, America has been very effective at cracking down on illegal tax-dodging by people who take cover under banking secrecy rules to hide their fortunes in offshore financial centers like Switzerland.

"The U.S. has been chasing banks. They have an awful lot of data and they've been arresting people, charging them and indicting them — very publicly indicting them and their lawyers. It's a very robust response and it's had a remarkable effect," said Christensen.

In 2009, a U.S. investigation into UBS resulted in a $780 million fine for the Swiss bank, which agreed to disclose more than 4,000 U.S. client names to the Internal Revenue Service and Justice Department.

Other European-based banks, including Credit Suisse and HSBC, and some of their clients, have more recently come under scrutiny in American probes.

Indeed, some banks have found offering offshore banking services to Americans such a regulatory headache that they stopped taking Americans on. HSBC's private bank said earlier this year it would no longer cater to American millionaires outside the United States.

"There are very few tax shelters left," says Martin Goldblum, chairman of the tax department at the New York law firm Troy Gould, which services super-rich clients.

LAST BASTION FALLS

The last real bastion of U.S. tax avoidance closed on September 9, the final day of an IRS tax amnesty window for Americans to declare foreign bank accounts.

This program, the second since 2009, allowed individuals who had not properly disclosed accounts and income earned abroad since 2003 to come clean, pay up their back taxes and penalties and file correctly in future.

The IRS said that as of September it had collected $2.7 billion from thousands of U.S. taxpayers. Among them were immigrants who kept holdings in their mother countries, people in the entertainment industry who earned money internationally, people with vacation homes in foreign countries and even some cases of people who had inherited accounts of which they were previously unaware.

Kaye Scholer's Unger stressed that these were often the reasons some Americans hadn't declared such income, countering a perception that all tax evasion was intentional and criminal.

"There's this feeling that it's only people who are trying to hide their money," he said. "On one of these accounts, I told the IRS agent, 'The reason they didn't tell you about the account is that they didn't know about it.'"

His client had inherited a $2 million account from a relative who had left Europe after surviving the Holocaust, and received a letter from UBS saying that as part of an investigation, the bank might release the names of U.S.-based account holders. That client ended up with a reduced penalty of 5 percent, or about $100,000, rather than the 20-25 percent typically charged.

"You can never say he's happy about that penalty," said Unger. "He came to the conclusion that these were the rules. He had an account at UBS. He didn't know if his name would be turned in. He wanted to clear things up and go forward with his life."

The highest penalty Unger has dealt with so far was $870,000, on an account of $4.35 million.

BRITISH BOLTHOLE

Billionaires in Europe — and in Britain particularly — can find it easier to escape the taxman's grasp.

Britain has the world's fifth-largest number of millionaires after the United States, Japan, Germany and China according to research published in June by Merrill Lynch and Capgemini.

But attempts by governments in Europe and Britain to crack down on tax evasion since the financial crisis have been noticeably gentler than those of the United States.

"The UK authorities are understaffed and don't have the latest technology," said the Tax Justice Network's Christensen.

In August, Switzerland and the UK did a deal to tax funds kept by British residents in secret Swiss bank accounts. It targeted just 5 billion pounds ($7.8 billion) in extra tax revenue, which critics say is a tiny fraction of what could be clawed back if the real extent of undeclared money was revealed.

"It's shocking. It's shabby," said Christensen.

Ronnie Ludwig, a partner in the private client team at accountants Saffery Champness, agreed the UK could have gone further if it was engaged in more vigorous pursuit.

"The actual figure that could come in ... would be significantly higher," he said.

The UK also permits residents with an often tenuous link to a foreign country to remain 'non domiciled' for tax purposes, which means they are not liable for money accumulated abroad, as long as it stays overseas.

This 'non-dom' status has helped turn London into an effective tax haven for thousands of rich foreigners, from Russian and Arab oil billionaires to global industrialists who earn most of their wealth outside the UK.