Anyone eligible for the tax rebate must file a return to get a check. For most of the some 130 million Americans eligible for a payment, that means filing a return as normal, the IRS said in a news conference. The tax agency will figure out the rebate for you based on your 2007 return.

But some lower-income taxpayers and recipients of Social Security and veterans' benefits aren't normally required to file returns. If they want their stimulus payment this year, they'll have to send in a tax return.

Asked how many more tax returns the IRS expects to process this year as a result of the stimulus package, IRS Acting Commissioner Linda Stiff said: "We project the number to be in the millions, likely in the 10 million to 20 million range, but we don't have any information available to us to say that with any degree of certainty."

Late filer? Wait for check

The IRS will start to mail checks in early May. While it's clear that taxpayers who file extensions or otherwise delay filing will not receive a rebate until after they do file, it's unclear how the IRS will determine the order of check distribution for taxpayers who file by April 15.

The agency is working on "an ordering mechanism for distribution of stimulus payments," Stiff said. "We're looking at that as we speak. There will be more to come on that. Our hope is that all of America will know when their checks are coming."

While filing a tax return is required to receive a rebate, stimulus payments will not be sent out with your regular refund.

Stimulus payments will not go out at the same time or in the same envelope or in the same distribution method as the tax refund," Stiff added.

About seven to 10 days before the rebate check is mailed, the IRS will send taxpayers notices alerting them of their total rebate amount. Taxpayers who signed up for direct deposit, however, are likely to receive the mailed statement at about the same time their rebate is deposited into their bank account, as using direct deposit will speed up the process.

The rebate statement is for taxpayers' records. If your tax situation changes in 2008, it's possible you could be eligible for a bigger rebate. At that point, you can use the statement to figure any additional rebate on your 2008 return. Taxpayers will not need to pay anything back if their changed situation next year would mean a lower rebate amount, according to the IRS.

Also, the rebate check will not be counted as taxable income. However, if you owe back taxes, the IRS will divert your payment to that bill before sending anything to you.

Eligibility and phase-outs

Workers who don't earn enough to pay income taxes but have earned income, including Social Security, certain veterans' and Railroad Retirement benefits of at least $3,000 would be eligible for $300, or $600 for couples filing jointly.

Taxpayers with children would receive an additional $300 per child, with no limit on the number of child-based rebates. That means a family with four children could enjoy up to $2,400, and a family with two kids could receive up to $1,800.

For taxpayers whose adjusted gross income tops $75,000 for single filers and $150,000 for married-filing-jointly filers, the rebate decreases by 5% for every dollar above the income limit, the IRS said.

Basically, that means the payment is reduced by $50 for every $1,000 of income. Generally, if you make more than $87,000 as a single filer or $174,000 as a joint filer, you will not get a rebate -- but those upper limits are different if you have children eligible for the rebate.

If you take the total possible rebate amount for your family, then subtract from that rebate amount 5% of your income above $150,000 ($75,000 for single filers), you can estimate your rebate.

For instance, a married couple with two children would be eligible for an $1,800 rebate check ($1,200 for a married couple plus $300 per child). But at an AGI of $186,000, their rebate would drop to zero, as 5% of $36,000 ($186,000 minus $150,000) is $1,800.

Already filed?

Taxpayers who already filed a return do not need to do anything else.

However, if you filed a return but reported less than $3,000 in income, you may need to file an amended return to be sure your qualifying Social Security or veterans' benefits are counted.

"Adding these benefits on an amended tax return will not increase an individual's tax liability but will establish eligibility for the stimulus payment," the IRS said.

See this IRS statement for more details.