It seems to me that deliberate misinterpretation by the Warren Court and its close decendents is what is involved in this issue. The Constitution clearly prohibits an established church like that which existed in England. The use of the term "establishment of religion" clearly refers to an established church, otherwise why use the word. As colonists, the founders were very familiar with English people being taxed to support a church they might or might not agree with. They did not want an established church in the U.S. One line in one letter from Jefferson is not significant enough to overcome the wording in the actual Constitution regarding this issue.
I believe this misinterpretation will be changed as time goes on just like the ridiculous view of the Second Amendment is now changing back to one of the right to self defense on the part of individual citizens not just a "militia" which was also a deliberate misinterpertation by the left.
Future decisions (given good results in the Nov. elections and Bush's ability to appoint better SC justices, which I know is NOT a sure thing) will reflect a change back to proper judicial interpretation of these two issues.
The current interpretation could indeed change with a future change in the make-up of the SCOTUS. However, it is simply the fact that in either case -- their interpretations become the law of the land -- just as the Constitution permits.
We have to remember other historical cases -- slavery was actually upheld by the Supreme Court in the Dred Scott decision -- because they couldn't find any specific language in the Constitution prohibiting it. That's a narrow reading -- an interpretation.
Some of us believe that your inability to find a prohibition of the state endorsing religion is also a narrow reading -- and we agree with the 9th's decision on this.