Any transaction in a tax haven is a sham  

From a New York Times article about tax cheats, who apparently bilk the government of $70 billion a year:

"The universe of offshore tax cheating has become so large that no one, not even the United States government, could go after all of it," said Senator Carl Levin, the Michigan Democrat whose staff ran the investigation.

Let's not get carried away here -- it should be obvious that we could spend as much as $70 billion (and possibly more if we consider enforcement and equity to have value in and of themselves) a year going after these folks and it would still be worth it. I guess that wouldn't be enough to invade and occupy Iraq, but it would sure fund a state-of-the-art auditing division for the IRS.