The stimulus rebate was technically an advance on your 2008 taxes. It is not taxable for federal or state of Iowa, but if you go to a tax preparer take it in as they will have to put it in the system because if certain circumstances have changed and you are due additional stimulus money it will come as part of your 08 refund.
For example if you were in college last year and your parents claimed you as a dependant in 07, but now have graduated and are claiming yourself in 08, you will have stimulus money coming your way. Also, if you had a child in 08, you will get stimulus money for them.
Fortunately, it doesn't work the other way and no one will owe money back to the IRS even if their 08 taxes wouldn't qualify them for the full amount they received.