Thanks for your comments, Ursula!
up> I am using nice menus with a zen 7.5 subtheme without problems.
I don't understand "zen 7.5". Is there a more complete name/version?
up> I would send you to the site, but the content is still up> confidential, thus protected.
Understood!
up> To test it for you, I just changed the default theme to zen (instead up> of the subtheme), and put the nice_menus menu into the navigation bar up> (main menu, menu-style: down). Everything looks fine on my site.
Strange ...
Further testing on my end turned something up: if I *log out* of admin mode, every thing works correctly! However, after logging in to an admin account, the menu reverts to that shown in the image sent previously.
Thinking it might have something to do with the (very useful) "admin menus", I disabled them (both the 'Toolbar' module in core, and the 'Administration Menu' additional module) and cleared all caches. Made no difference.
So, all I can see is this: if I an not logged in, the menu is fine; if I login to an admin account, the menu is multi-lined and popups don't work.
The Nice Menus "issue queue" has this related "closed" entry:
http://drupal.org/node/1528900
But its solution (remove Simple Menus) doesn't apply to my setup since I'm not using that module.
Any other ideas?
Thanks,
-Kenneth