Given the precedents at the college level and court rulings on public aid to parochial schools, I don’t think that vouchers for religious schools would necessarily be unConstitutional.
Otoh, I strongly believe that the whole vouchers issue is really at the root of the religious right’s current emphasis on ID and Creationism.
As Rush Limbaugh is fond of saying, follow the money trail.
ID is a hot-button wedge issue that serve to push susceptible parents further from the public schools. There are many other such issues, and much better ones, but this one is relatively simple and specific and bypasses such messy controversies as multicultural revisionism and teacher tenure.
ID serves to increase the pressure for vouchers. These, in turn, would bring billions in loot into the coffers of the religious organizations that are best prepared to offer creationist education; the same organizations, obviously, that are beating the drum for ID.