Two chances to qualify for rebate
Note that you have two chances to qualify for the rebate/credit. If you qualify based on your 2007 tax return, you get the "refund". If you do not qualify based on your 2007 return, but qualify based on your 2008 return, you get to take a credit against your 2008 taxes.
This is implied in the language on the actual law.
Search Results - THOMAS (Library of Congress)
The law's language is, like most of the tax law, only vaguely related to English.
Here it is in something a bit closer to English.
http://www.house.gov/jct/x-16-08.pdf
"Rebate checks
Most taxpayers will receive this credit in the form of a check issued by the Department of the Treasury. The amount of the payment will be computed in the same manner as the credit, except that it will be done on the basis of tax returns filed for 2007 (instead of 2008. It is anticipated that the Department of the Treasury will make every effort to issue all payments as rapidly as possible to taxpayers who timely file their 2007 tax returns. (Taxpayers who file late or pursuant to extensions will receive their payments later.)
Taxpayers will reconcile the amount of the credit with the payment they receive in the following manner. They will complete a worksheet calculating the amount of the credit based on their 2008 income tax return. They will then subtract from the credit the amount of the payment they received in 2008. For many taxpayers, these two amounts will be the same. If, however, the result is a positive number (because, for example, the taxpayer paid no tax in 2007 but is paying tax in 2008, the taxpayer may claim that amount as a refundable credit against 2008 tax liability. If, however, the result is negative (because, for example, the taxpayer paid tax in 2007 but owes no tax for 2008, the taxpayer is not required to repay that amount to the Treasury. Otherwise, the checks have no effect on tax returns filed for 2008; the amount is not includible in gross income and it does not otherwise reduce the amount of withholding."