-is exposure of any kind OK?
Yes, if you create an effect that uses only sleights in the public domain, you can give it away for free. If you want to make a video tutorial teaching public-domain moves and effects you can, because you're not harming the creators of an effect, plus, you can find that information anywhere anyway. Though I don't see why you would want to waste your time teaching a move that's already all over the internet.
-what if you figure it out from a demo video?
You should not perform the effect, or support the artist and buy the effect. Though I have wondered lately how much goes to the creator, and how much goes to the vender.
1. after watching it a million times with the intent to discover the method
That just isn't right. The artist put is time and most-likely money into put the effect out there. Plus, you're not going to get nearly enough insight as to how to perform the effect anyway.
2. the first time, and it just hit you
You either buy the effect, or don't perform it.
3. you get an idea for how you would do it, but you dont think thats how its done
I'm a bit split on this one. The artist came up with the idea, and you came up with a different way of doing the same thing. It's similar to stealing an idea, just not the method. I think it's sort of okay to perform your own version of an effect if you haven't bought it. Then again, if you haven't bought, there's really no way of telling that your method is different.
- what about cardistry? its not built upon secrets.
This is tougher. Obviously you'll get more info about how to perform the flourish (I myself can't understand how to do a particular cut/flourish without being walked through it) You should support the artist if you plan on performing the flourish. You wouldn't exercise using a commercial for an exercise video. (Maybe that isn't a valid point, I'll have to think about this one more)
- performance and patter rights?
I'm not 100% sure, but I think you can get some sort of exclusive television performance right. My Stigmata DVD, and my Healed 'n' Sealed booklet (I think, I don't have HnS anymore) said that John Doe has exclusive television performing rights.
A lot of people say that you should always come up with your own presentation, and not use the one taught in the Book/DVD/Download/Booklet. I consider that BS. You always need a good presentation, and often the one given in the instructions is a little lacking, but when the presentation is good, and you can't think of anything better, go for it.
- how much can be said for the legality of the issue?
I saw a slide show about copyright, fair use etc. in a computer class I took last year, and as far as I known, you can't copyright physical movements, only concrete things. Then again, there's the television performance right thing.
- public libraries? if its there the library has permission for it, but should we use it anyway?
If a public library has a magic book, the book itself, or the moves disclosed in it are most likely public domain, which means there is nothing wrong with performing effects and sleights out of a library book.