Thank you! I think I found it, following your pointer:
(quote)

You must add back to federal taxable income any amounts previously deducted and were not used to pay for the qualified higher education expenses of the designated beneficiary, unless the withdrawal was made without penalty under section 529 of the Code due to the death or permanent disability of the designated beneficiary.

So, since my state only allowed deductions starting in 2006, I believe I need to dig out my 2006, 2007 and 2008 state tax returns, find the amounts deducted on each of these, then add all of those amounts together, then add the total to my federal AGI when I file 2009 taxes. Does that sound correct?