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.
Directly from the IRS website. You should seriously check it out....the info is out there in black and white.
Stimulus Payments: Answers to Frequently Asked Questions