You're probably wondering how there could be any tax planning to do for your rebate check for the rest of the year, if they're already being mailed in May for most taxpayers. Well, the reason is that in reality, this rebate check, based on your 2007 tax return, is technically an "advance payment" for a rebate check based on your 2008 tax return! Given the timing of when the Economic Stimulus Act passed, Congress wanted to get payments out to everyone as quickly as possible, but the end result is a tax planning opportunity for many.
According to the technical language of the tax bill, individuals will essentially be able to receive a rebate check for the better of the amount due based on their 2007 tax return or their 2008 tax return. If you file your 2007 tax return and receive the maximum rebate amount, there's nothing more to do. However, if you do not receive your maximum rebate in 2007, you have a second chance to maximize it with your 2008 tax return, which means that tax planning opportunities remain!
What kinds of strategies would be appropriate? Well, if you phased out the rebate because your income was too high, it's a second opportunity to phase out less or none of the rebate for the 2008 tax year. Of course, it's not necessarily wise to lose economic value just to get a tax credit, but if you have control over the timing of your IRA withdrawals, Roth conversions, capital gains recognition, or other tax event, deferring it to 2009 may help you maximize the tax credit, in addition to the overall benefit of tax deferral. Since the credit phases out at 5% on the dollar, that effectively means that an individual in the 25% tax bracket who is also phasing the rebate check phaseout has an effective rate of 30% if they defer income. That doesn't mean it will always be right to defer, but the effective impact is higher.
This may also mean that a change in circumstances could create a rebate check opportunity in 2008. For instance, a child born by the end of the year would be another dependent eligible for the child portion of the rebate credit (assuming it wasn't entirely phased out). A couple who gets married in 2008 and files a joint tax return for the first time may find that they are eligible for more of a credit as a married couple in 2008 (with the higher married couple phaseout) than each was individually in 2007 based on their individual incomes. This can be particularly true if one new spouse had higher income (above the individual phaseout threshold), while the other spouse had much lower income - the end result in 2007 may have been a full credit for one but not the other, but as a married couple in 2008 their joint income may still be eligible for the full credit.
Again, if your rebate credit was already fully maximized with your 2007 tax return, there is no planning left to do for the credit. However, if it wasn't, then it's possible that you could be eligible for more of the credit next year, based on your 2008 tax return - so start planning now!
The IRS has proivded several helpful resources for this as well, including a page with basic information about the stimulus payments, an additional page of Q&A discussion, and an online calculator for determining the estimated amount of your 2007 rebate amount.