Economic Stimulus 2009

The tax rebate payment is an advanced tax credit for tax year 2008 return which will be calculated on your 2008 return when filing at a rate of 6.2% of your earned income

An advance credit means that its a refundable tax credit of up to $400 for working individuals and up to $800 for working couples.
The tax credit is calculated

The Irs tax rebate check will not be sent automatically, so most taxpayers are not going to receive their stimulus check in mailbox. It will be paid through a reduction of income tax withholdings starting no later than April 1 of this year.

H-R tax calculator
Use the 2008 Tax Estimator to calculate your refund or how much you'll owe.