I went back and looked it up, Jerome. In 2001 GW managed to get his tax cuts passed. It was mainly targeted to the rich, but to show the little people that they, too, got some small benefit, he mandated that the IRS should send out rebate checks in lieu of the lower rates. What presumably would have happened without the checks is that the average taxpayer would have seen slightly less tax to pay the next April due to lower rates; he'd have gotten back a bigger refund. With the checks, he got his piece earlier, praise GW, but had to pay that amount back in April, along with the smaller tax due. It was, indeed, slight of hand: the decrease in taxes was real enough, but the advance was just that - an advance, due to be reimbursed when the taxes were due.
This one, in 2008, is not an overall tax rate decrease. It is a one-time credit. When you get your refund back it will be $600 (or some such) larger than it would have been. It actually has nothing to do with the taxes; that is just an easy way to figure and distribute it.
They are ADVANCING the refund of credit.
No advancing to it. You get the credit, either as less taxes or more refund; you don't have to pay it back (at least, not in the usual sense; yes, certainly, you will, sooner or later, pay for it with more taxes).
Yes, you can finagle your deductions and move the withholding back and forth. In general, you get to pay a penalty if you don't withhold an amount greater than or equal to 80% of your final tax bill, and the gov gets all the interest on overpayments ("refunds"). 2nd law of thermo in action. But it's not a scam; it is (if it gets passed and implemented) a bona fide credit in the neverending ledger sheet between you and Uncle, unlike that check in 2001.