According to the IRS, the economic stimulus payment is not taxable and will not reduce your 2007 or 2008 refund or increase the amount you owe when you file your 2008 return.
You should keep a copy of the IRS letter you receive later this year, listing the amount of your payment. In the event that you do not qualify for the full amount this year but you do next year, you will need to have the letter as a record of the amount you previously received.
For more information on the Economic Stimulus Payment, visit www.irs.gov.