When a term becomes popular enough, it will be hijacked by marketing to stretch its definition to include whatever pleases them.
Look at AJAX for example, about a year ago a company release a product that had "Ajax" on its name (I think it was AjaxWriter), but it didn't use AJAX at all! They made so much noise that they've got about 500K users visiting their site on the first week. Very impressive.
I'm a technologist, and I like things well defined. AJAX for me is AJAX (Assynchronous JavaScript and XML). It won't buy any marketing talk about FLEX being AJAX, or Flash being "Ajax-y", or XUML, or whatever.
But I'm not a marketing person and I see what this is appealling.
John Cook is talking/asking today about Zillow being a wiki or not. Heck, I don't even consider Wetpaint a Wiki. A Wiki for me is what Wikipedia is built on top of. Anyone can sign up and start editing. When you need authorization, then it is not a wiki anymore, it is SharePoint or any other CMS tool out there.
Here is my strict Wiki attributes:
It has that funky Wiki language;
It allows anyone to edit its content;
It automatically links to other definitions;
Here is my strict Blog attributes:
It is a list of text in reverse chronological order;
A lot of people, like Dave Winer, has an even more restricted view of what a blog is (like having comments and syndication).
Now, let's not confuse Technology Terms with Marketing Terms.
UPDATE: Scot French points out that I didn't answer explicitly John Cook original question. No, Zillow is not a Wiki on my view.
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January, 2008