Public education is estimated to cost $10K/student. Private education is estimated to cost $5K/student. If more students were to go to private schools, this would decrease the average daily attendance in public schools, thus decreasing general funding to education without affecting curriculum.
Don't forget that Article IX, Section 1 of the Idaho Constitution mandates that the state provide a thorough public education to its citizens.
Make the voucher equal to the property taxes paid for public schools. Those who choose to send their kids to private schools are helping the economy with the tuitions they pay.
Ideally people should be able to go to college for free, such as in Sweden. But since FY10 is proposing huge education cuts, I have a feeling this is another issue that will get placed on the back-burner.
Good idea though.