This is nonsense. The stimulus rebates are straight cash giveaways, not loans against 2008 tax liability.
From the IRS website FAQs about the rebates -
Q. Will the payment I receive in 2008 reduce my 2008 refund or increase the amount I owe for 2008?
A. No, the stimulus payment will not reduce your refund or increase the amount you owe when you file your 2008 return.
Doesn't get clearer than that...