That’s perhaps the boldest declaration in the joint statement from G-20 world leaders meeting today in London.

The communiqué, which pledges $1.1 trillion in global stimulus spending and tighter supervision and regulation of the global economy, did not commit the leaders to another round of stimulus spending as President Obama wanted, nor cross-border financial regulation as Germany and France hoped.

But it does announce a new tool of transparency: the formal blacklisting of tax haven countries that have not agreed to international information sharing agreements. Fast money operations like  AIG haved use these countries to claim they don’t have to pay  U.S. taxes. Estimates of lost tax revenue range from $40 billion to $123 billion annually, according to recent Treasury Department study cited by The Wall Street Journal. The communique strengthens Obama’s efforts get capture these revenues to pay for his ambitious domestic agenda.

Their recent tax woes — by which we mean their getting caught not paying their (or their spouses’) taxes — mean that the IRS is probably right about (or even underestimating?) the extent to which Americans — both individuals and corporations — cheat on their taxes. (In its latest tally, the IRS estimates that the 2001 tax gap — the chasm between taxes owed and taxes actually collected — was $345 billion.)

Why is that good news? Because much of that could be retrievable.

As Te-Ping Chen at The Center for Public Integrity points out this week, every dollar the IRS spends on tax enforcement returns five dollars. Chen also offers a few reasons why the IRS hasn’t closed the gap in recent years:

One reason is a slump in IRS staffing. Over the past decade, the number of agents that perform audits has dropped by over a third. Meanwhile in 2007, in what the Syracuse University-based Transactional Records Access Clearinghouse calls a “historic collapse,” only 26 percent of corporations holding at least $250 million in assets had their books inspected — compared to more than 70 percent in 1990. (The fact that Congress outsourced debt collection to private agencies in 2004, costing the government $37 million more than such agencies manage to collect, hasn’t helped either.)

Sen. Kent Conrad (D-N.D.), who chairs the Senate Budget Committee, has been screaming from the rafters for years about how Congress needs to find some way to close the tax gap. Daschle & Co. might just provide the impetus for lawmakers to start trying in earnest.