Simon's questions were specifically about how it is that the license permits us to require the inclusion of an HTML linkback in the invariant sections. In my first response, I answered that question, though of course more details may come out in the course of further discussion.
However, here I want to explain why the attribution requirement is important and a good thing.
One of my goals for the encyclopedia projects that I have started is that they be *widely used*. Another goal is that my ideas of openness, fairness, objectivity, community consensus become the standard by which encyclopedias are judged.
What we want to see is Yahoo, AOL/Time Warner, Disney, Google, Microsoft, Altavista, Lycos, etc., all decide to adopt our encyclopedia as the foundation for their own-branded encyclopedia products. But when they do so, we want them to link back to the original project, so that we can ensure that we remain the "canonical source" for our own community works.
I pick Microsoft for my example, because they are unpopular.
Imagine this scenario -- Microsoft takes our content, and makes their own Microsoft Online Encyclopedia from it. Their version is on their web pages and is modified by them. It is redistributable and modifiable by others, as it has to be under the license. But (!) imagine that we do not require them to link back to us. And (!) imagine that their version is not editable by anyone but them.
Then we end up with a situation in which their propaganda can be inserted into the Wikipedia, and their readers have no means to find us, the original source and the ultimate corrective to bias. I think that's undesirable.
I don't mind Microsoft taking our content, republishing it with their own propaganda inserted. That's fine, that's something we accept under the FDL. But I want them to include the invariant section -- the invariant section which leads people back to the original, so they can compare for themselves, and hopefully join our community.