When it comes to taxing a major Facebook founder’s windfall, Sens. Chuck Schumer and Bob Casey are so eight years ago.

The legislators held a press conference today to unveil a bill they are calling the “Ex-Patriot Act” (“Expatriation Prevention by Abolishing Tax-Related Incentives for Offshore Tenancy”), aimed at preventing wealthy Americans from dropping their citizenship in order to avoid taxes. Last week, it was discovered that Facebook co-founder Eduardo Saverin had expatriated in September of last year.

“Eduardo Saverin wants to defriend the United States of America just to avoid paying taxes,” Schumer said. “We aren’t going to let him get away with it.”

The senators' bill seeks to apply a mandatory 30 percent capital gains tax on anyone who abandons the United States, and it would renew an existing law, rarely enforced, that bars those people from setting foot on U.S. soil.

If the bill becomes law, it would return tax policy on expatriating citizens to a more punitive approach than existed before 2004.

Before that year, certain tests were applied to expatriating citizens’ finances to determine whether they were leaving for the “principal purpose of tax avoidance.” If the person was worth at least $2 million, or had an average income of $145,000 or more in the two previous years, they could be taxed on their U.S. gains for the next 10 years.

In the mid-1990s, the IRS noticed that some wealthy individuals were working around the principle-purpose definition by migrating their trust funds abroad. “I asked whether they were doing this because the beneficiaries were expatriating,” recalls Michael Pfeifer, former chief counsel at the IRS, and now a partner at Caplin and Drysdale, a Washington, D.C. law firm.

Out of Pfeifer’s investigation came the current policy, which was updated to its present form in 2008. Known as “mark to market,” it is a cleaner process that imposes an exit tax on the expatriator’s worldwide assets at the time citizenship is relinquished.

It's not likely that Congress will take up the Schumer-Casey bill, not only because the current policy was enacted so recently, but because it is widely seen as sensible and fair. “Tax law is not supposed to be a fence that keeps people in the United States,” says Pfeifer.

Under the mark-to-market rules that pertain today, a departing citizen files Form 8854, attaching a balance sheet of all of his or her assets. The feds effectively pretend that any illiquid holdings have been sold on the last day the taxpayer was a citizen. The IRS then reckons the capital gains tax on this “notional” sale.

By expatriating, Saverin, who owns about 4 percent of Facebook, has by no means avoided taxation altogether. His statements in recent days indicate that he’s willing to pay this exit tax fully. "I am obligated to and will pay hundreds of millions of dollars in taxes to the United States government," he said in a statement released today.

(He doesn’t have to do so immediately: He could put up a bond or letter of credit for what he owed until such time as he actually sold his assets.)

But despite Saverin’s denials, experts have noted that the move will benefit him in several ways. “It’s ingenious,” says Kevin Packman, chief of offshore compliance at the law firm of Holland & Knight. “He will save a bundle when selling stock after the IPO.”

Most obviously, Saverin will save by taking his capital gains hit against the presumed value of his shares in late September, when Facebook was valued at about $90 billion. If Saverin had hung around until after the IPO, which looks like it will improve the company’s valuation to some $112 billion, his capital gains exposure would be significantly higher.

Second, Saverin now seems to have joined the tax jurisdiction of Singapore, which has no capital gains tax at all. When he sells shares or takes dividends on his Facebook holdings (if any), he’ll get to keep it all.

That move alone, Schumer and Casey charge, will save Saverin some $67 million. And they apparently haven’t counted in Singapore's much simpler reporting requirements and regulations, saving Saverin untold amounts in legal and accounting fees.

In addition, since there was no public market for Facebook shares at the time he filed his 8854, it was up to Saverin to estimate their worth. Pfeifer says that expatriators often discount the value of their assets as much as 35 percent. That “give” in the share price will evaporate as soon as the IPO establishes a firm market for Facebook shares. (The IRS is free to challenge Saverin’s estimate in an audit, and if they do, Saverin could negotiate a mutually agreeable price.)

Saverin also may have also been looking ahead to the end of this year, when the Bush tax cuts expire. In any “Taxmaggedon” deal between Congress and the White House over spending and taxes, the Democratic side will be arguing to bump up the capital gains rate—Schumer and Casey didn’t pluck the 30 percent rate in their bill out of thin air. Says Pfeifer. “He probably didn’t want to take the chance of paying a higher rate.”

The “mark-to-market” standard won’t change if the senators get their bill passed—they’ll only make Saverin’s decision slightly less ingenious. But by insisting on refusing him visits to his adopted country (Saverin was born in Brazil), the senators clearly mean to re-instill a sense of outrage that died with the old policy.

Or almost died: One remnant of the former law’s judgment was the quarterly publication of the names of those who renounce their citizenship in the Federal Register. It was Saverin’s appearance on this list of shame that revealed his new citizenship status.