> If we do end up maintaining the cookbook, we should just make the
> license clear on every page.

This is a good idea. In my experience, most sites like this have unclear
licenses.

> Perhaps we could even annotate old code as
> having only the old license, but include a user agreement that licenses
> code under multiple licenses so that users have the most flexibility
> possible.

Also worth considering! This possibility didn't occur to me at all.

>     Here's the Stack Overflow licensing policy:
>     Creative Commons CC-BY-SA
>
> I would trust their approach.  They are such a huge site, with so many
> users, that I assume that they must already have dealt with any possible
> issue that could come up.

It would indeed be reasonable to assume so, but I browsed some of that
discussion once and got the impression that they don't really know what
they are doing (but neither does anyone else necessarily - this is
apparently a very murky area, both the legal interpretation and the
social conventions surrounding re-use).

IIRC they used to have everything under MIT License with some custom
attribution clause added (sounds kind of weird). I got the impression
that there isn't a clear sense of direction there.