For example, a State may lend to parochial school children geography textbooks that contain maps of the United States, but the State may not lend maps of the United States for use in geography class. A State may lend textbooks on American colonial history, but it may not lend a film on George Washington, or a film projector to show it in history class. A State may lend classroom workbooks, but may not lend workbooks in which the parochial school children write, thus rendering them nonreusable. A State may pay for bus transportation to religious schools 10 but may not pay for bus transportation from the parochial school to the public zoo or natural history museum for a field trip. A State may pay for diagnostic services conducted in the parochial school but therapeutic services must be given in a different building; speech and hearing "services" conducted by the State inside the sectarian school are forbidden, Meek v. Pittenger, 421 U. S. 349, 367, 371 (1975), but the State may conduct speech and hearing diagnostic testing inside the sectarian school. Wolman, 433 U. S., at 241. Exceptional parochial school students may receive counseling, but it must take place outside of the parochial school,12 such as in a trailer parked down the street. Id., at 245. A State may give cash to a parochial school to pay for the administration of state-written tests and state-ordered reporting services, but it may not provide funds for teacher-prepared tests on secular subjects. Religious instruction may not be given in public school, but the public school may release students during the day for religion classes elsewhere, and may enforce attendance at those classes with its truancy laws.All of these inconsistencies could be easily resolved with vouchers and tax credits! update: In an ideal world, that is. Not inside the actual world we live in.
WALLACE, GOVERNOR OF ALABAMA, ET AL. v. JAFFREE ET AL. APPEAL FROM THE UNITED STATES COURT OF APPEALS FOR THE ELEVENTH CIRCUIT No. 83-812. Argued December 4, 1984-Decided June 4, 1985. (110-111).
update 9/29/2012: Rehnquist compiled the list above to support his argument that the Court's 1971 "Lemon test"* had resulted in unprincipled and inconsistent decisions.
...[T]he wall [of separation between church and state] idea might well have served as a useful albeit misguided analytical concept, had it led this Court to unified and principled results in Establishment Clause cases. The opposite, unfortunately, has been true; in the 38 years since Everson  our Establishment Clause cases have been neither principled nor unified. Our recent opinions, many of them hopelessly divided pluralities, have with embarrassing candor conceded that the "wall of separation" is merely a "blurred, indistinct, and variable barrier," which "is not wholly accurate" and can only be "dimly perceived." Lemon v. Kurtzman, 403 U. S. 602, 614 (1971); Tilton v. Richardson, 403 U. S. 672, 677-678, (1971); Wolman v. Walter, 433 U. S. 229, 236 (1977); Lynch v. Donnelly, 465 U. S. 668, 673 (1984).* Lemon v. Kurtzman, 403 U. S. 602, 612-613 (1971)
the Establishment Clause
Free Exercise Clause
Wallace vs Jaffree
the Lemon test
Lemon v. Kurzman
Justice Rehnquist lists "unprincipled" and "inconsistent" decisions
Justice Rehnquist on school prayer and the Constitution
Jefferson's "wall of separation"
Justice Burger's dissent in Wallace v. Jaffree
Benjamin Cardozo on metaphors in law