Allright, I took a deeper look at the whole thing and what it comes down to is this...the tax rebate that they're pushing through is essentially an advance on a tax *credit* for your 2008 taxes.
What will happen is, they'll issue you a check based on whether your 2007 tax return indicates that you would be eligible for the 2008 tax credit.
When it comes time to do your 2008 taxes, some people who were not eligible for the credit based on their 2007 return may be eligible for more/less of the credit based on their 2008 taxes (say, if you have additional/fewer children or you make less/more money).
If they issued you a rebate based on their estimation of your tax credit eligibility, and your actual tax credit is less, you get to keep the difference. If they issued you a rebate based on your tax credit liability and your actual tax credit is more, you'll get the difference.
Sooo...if you didn't have any kids in 2007 and can manage to pop one out by the end of 2008, that might be worth an extra $300 to you. However, if you do have kids in 2007 you can safely kill them in 2008 without endangering your $300 tax rebate.
None of that strictly speaking has to do with your refund, rather again it has to do with your 2008 tax liability, and whether the estimation of the 2008 credit being advanced to you in 2007 is accurate based on your 2008 tax return.