Professor Eugene Volokh predicts a Supreme Court victory for Arizona's Tuition Tax Credit Program (the petition for hearing was granted just today), and sees it as likely building on the court's ruling (in Zelman) that school vouchers are constitutional.
[T]he Court’s decision here may be pretty significant, and may strengthen the Zelman principle to make clear that these sorts of programs are indeed constitutional. Justice O’Connor joined the Zelman majority, but wrote a concurrence that could be read as expressing a somewhat narrower position than the majority’s… I suspect that Chief Justice Roberts is at least as supportive of religious groups’ participation in a wide range of evenhanded funding programs as Chief Justice Rehnquist was, and that Justice Alito is more supportive of it than Justice O’Connor was. So unless the case is decided on standing grounds, I expect the opinion to be a pretty solid win for the Arizona program and for other such programs more generally (so long as they are legally equally open to secular and religious institutions).