I lied when I said:

I’m temporarily freezing development of Thistle as of the date/time of this post; the rest of my work today will be towards getting the development release ready.

Things always come to light when you're preparing for a release. I've been doing some 
additional coding to deal with a couple of issues which only just came to my attention:

• We now allow using a ”?cat=whatever” URL to display the contents of hidden categories (i.e. categories listed in $ignore_folders). This is useful for keeping entire categories private, but still being able to view their index pages. In this situation, the hidden category will show up in the category list; it doesn’t make sense to hide it when you’re already viewing it! For example, take a look at my hidden Preview Area category.
   
• Root-level categories are now indented in the category list. The only un-indented item in the category list is the top-level category.
   
• The $topLevelTerm variable in CatList.php is now overridden by the category-name file at root level, if one exists.
   
• The category list now really always sorts the root level at the top.
   
• We now have a new option, $show_errors, which determines whether error messages are displayed for minor errors (such as no posts being found in the current category). Major errors (such as not being able to access the $datadir) are always reported. This can naturally be overridden on a per-category basis, for example if you want a category to show just static content at the top of its index page, with no posts displayed: ordinarily, you’d also get a “No entries found” message below, but by setting $show_errors = 0 in your category-prefs file for that category, you can suppress the message.

Hopefully we're still on track for a release very soon, though it may be delayed until tomorrow. Thanks for your patience!