It won't impact your 2007 taxes at all.
If things work out the way it's been implied so far, the plan is that the 2008 tax rates are reduced on the first $6,000 of earnings for individuals (and $12,000 for couples) from 10% to zero. (That's why you see those $600 and $1,200 amounts being mentioned.)
This tax rebate is an advance on that 10% tax reduction. So, all that's happening is that the government is giving you your tax "refund" early. You've been given the benefit of getting this money this year rather to wait for it next year. It's a net gain/loss of $0 for those people getting it. (Spend it wisely.)
If, after figuring out your 2008 taxes, it turns out that you received too much of a tax rebate, the government does not plan to make you repay any part of that advance rebate. If after figuring out your 2008 taxes, it turns out that you would have been entitled to more of a rebate (than what your 2007 AGI allowed) then, they'll pay you that additional amount next year.
This is a win-win situation for anyone who qualifies.