If you have moved since you filed your 2007 tax return and expect to get a stimulus rebate, you need to file a Form 8822 (Change of Address) with the IRS. The form can be downloaded from the IRS web site or ordered from the form line (1-800-829-3676). If you have moved, the post office is not suppose to forward IRS checks. They are returned and held until the IRS gets a new address from you.
With the first checks to go out in 3 weeks, this is not something you want to procrastinate any longer. The second page of Form 8822 hold the instructions and mailing addresses based on your resident state. And do use all of the zip code. The +4 identifies your letter as a change of address and gets it to the right department more quickly.