Thanks - that seemed to do the trick. I think the program was confused by the fact that my itemized deductions were greater than the standard deduction, but smaller than the standard deduction + the $600 I was allowed to take for charitable donations.
When I went back through the deductions screen and told it to take the standard deduction even though it was lower than the itemized deduction, it correctly filled in $600 in box 12b and let me complete my return.
