The danger is not merely resegregation, although that is a valid concern. It is teacher stratification. Most teachers unions insist on voluntary transfers based on seniority. The poorer and less white a school gets, the faster teachers with seniority bail out. Eventually you get the educationally lethal combination of poverty, racial segregation, and teachers who are either rookies, ineffective or burned out.
Parents deserve the opportunity to get their kids out of schools like that.
The biggest mistake liberals make is assuming that vouchers are intended for people who want to send children to elite private schools. The people who most want vouchers want to send their kids to parochial schools and small private schools yet to be born. I don't see how those of us who consider ourselves liberals can continue to argue that the children of the poor must be prohibited from seeking better education in different schools because it is unfair to the people who are failing to educate them properly. That is profoundly wrong-headed. President Bush has laid aside voucher plans targeted to the poor, largely because they are so misunderstood by both conservatives and liberals.