As last-minute income tax filers worry about whether they can finish their Form 1040 in time to get it to the post office, or on its way electronically, some other folks are facing an agony all their own.

They have until midnight tonight to decide whether to confess that they have been hiding money offshore and using a credit or debit card to make use of it.

That's the deadline the Internal Revenue Service set when it began running a partial amnesty program for such folks in January.

Those who do come forward will have to pay back taxes, interest, and certain accuracy-related penalties, but will not face criminal sanctions and other civil penalties. "If you have unreported offshore income, this is as good a deal as you'll ever get," acting IRS commissioner Robert E. Wenzel said in a statement. The tax agency has won court orders allowing it to see the records of Visa, MasterCard and other cards issued by banks in a number of popular tax havens. It has already started following this paper trail to American taxpayers it considers suspicious.

Dale Hart, the IRS deputy commissioner overseeing the amnesty, said in an interview that the amnesty already has produced millions of dollars in new revenue from "a good number" -- she wouldn't specify how many -- participants.

One of the demands the agency makes is that participants explain "the arrangements they have used and who put them into it."

Those who don't come forward and are ultimately caught will face the full range of civil penalties, the IRS says, including the civil fraud penalty, which is up to 75 percent of the unpaid tax liability attributable to fraud.

To qualify, taxpayers need only get their basic information -- name, address and the like -- to the IRS, postmarked by midnight tonight, Hart said. The details can be worked out later.