DH,

  The wiki that you want is MindTouch Deki.  Below I've addressed each of your requirements.

Mixed public site with some content viewed only by registered users.

With MindTouch Deki's permissions you can have different roles for users and different permissions for pages.  This will allow you to have restricted content that is only available for some users. 

Form based, or even ajax-based data entry and editing of many classes of data (for example, one class might entail 2 picklists, a date, and freeform text, with 2 additional fields populated based on the page content), preferably in place.

You can put a form on a page and have it post to the wiki and create content the way you want to.  This would require some coding as it doesn't ship with a form to post to the wiki. 

Want to associate data entered with multiple pages, sort of like through widgets that show dynamic content.

MindTouch Deki excels at this.  With MindTouch Deki you can easily do mashups on data that is relavent to each other.  Take a look at our SoCal fires page which aggregates from sources like Flickr, Google, Youtube, RSS feeds, Twitter and more: http://wiki.developer.mindtouch.com/Com … SoCal_Fire 

Templated page creation with standard sections, and tools appropriate to naive users.

MindTouch Deki has an easy to use interface for editing and adding new content.  It also has the Template features which allows you to mockup page layouts that can easily be inserted into pages.

Would prefer hosted solution, or a solution where there's active maintenance esp. for code to protect against spam, porn, script kiddies, etc.

MindTouch Deki has Managed Hosting options so that you don't have to worry about the upgrades or dealing with Spam. 



If you have any other questions about MindTouch Deki please feel free to contact us: http://wiki.mindtouch.com/support