Dual licensing -- other wrinkles

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Dear all,

The discussion has been very enlightening. Some other wrinkles I thought of:

As the author of a work, I am free to license it as I will, even offering it under 2 or more licenses. Similarly I can require all contributors to allow me to do the same thing.

Now:

1. Can I simply write a preamble in my headers saying "if you didn't pay for this, it is licensed under GPL; if you did pay for this, you can either choose GPL or (unnamed commercial license)"?

2. I'm not interested in the complexities of collecting sublicensing and subsublicensing fees -- all I want to do is, if you pay for it, you can use it any way you see fit, including except that you can't sublicense it as anything else but GPL. Would the header preamble handle this? Is it sufficient for the other license to be some sort of permissive non-reciprocal license like CPL, BSD, etc., or do I have to put more teeth into it?

3. Suppose at some later stage, I discover another GPL'ed derivative of my work in the wild. Does the fact that I have dual license mean that if the other author says, "I don't want to submit this code back to you under your dual license", I cannot then incorporate his code back into my dual licensed code base?

Thanks in advance for your answers.

Cheers,
Glen Low, Pixelglow Software
www.pixelglow.com
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