Thanks for checking.  Yes, I used the developer toolbar to look for the curid span.  I think I see why I'm confused: I had clicked on a namespace start link in the indexmenu -- it doesn't have the curid span.  But other page links do.  Here are two examples for comparison:
  * No curid here: http://www.zoology.ubc.ca/~rikblok/wiki/doku.php?id=math:pythagoras:start
  * curid is here: http://www.zoology.ubc.ca/~rikblok/wiki/doku.php?id=math:shared_gas
If you check the CSS of the indexmenu in the sidebar you should see the start page doesn't have a curid but the other one does.

Maybe it would be best to just catch start pages and wrap them in a curid span manually?  Then the user can customize curid styling as you suggested.