Morally, don't think it's okay to teach ID/creationism anywhere, including religious schools. That's because ID/creationism as they exist in the US aren't a bunch of philosophical beliefs about the unknowable or a set of reasonable criticisms about prevailing scientific theories -- they're pure bullshit based on a web of intentional fabrications. Morally, socially, culturally, these ideas are bad. Nobody in the ID movement -- not the Discovery Institute, not Dembski, not Behe, not Davis and Kenyon, and most certainly not Ben Stein -- wants to teach children to think critically. They want to teach children not to think. And that's unconscionable regardless of whether you're doing it in a public school, in a private religious institution, or at home.
This is why it's worthless to engage with cdesign proponentsists(*). You can't have an honest discussion with someone who is determined to avoid any kind of honesty at every turn.
Now, the people who just believe the lies of cdesign proponentsists... well, it is useful to try to engage with them. However, no amount of naked facts will sway them, no matter how incontrovertible they are. Neither will a philosophical discussion about why evolutionary biology or Big Bang theory or whatever might still permit the existence of God. Because what you're dealing with, fundamentally, is the systematic cultural acceptance of something that many know has to be a lie because it's considered a fundamental component of their entire worldview [http://slacktivist.typepad.com/slacktivist/2008/10/they-need-help.html]. And getting past that is fucking hard.
-- Alex
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* - If you're not familiar with the term, you can read about it here [http://en.wikipedia.org/wiki/Of_pandas_and_people#Pandas_and_.22cdesign_proponentsists.22] or here [http://www2.ncseweb.org/wp/?p=80].