Man! What a waste of time.
Turned out that having curved corners and other HTML5 goodness applied using the PIE method to get them to work in IE lt9 versions was creating new siblings in the DOM, of course. Finally looking at IE generated source showed new “<css3-container>” elements next to <a>, <div> etc, which were messing up “next” selector method from jQuery.
To fix, just removed the curved-corner classes and presto chango, Robert remains your mother’s brother. Live and learn.
Simple demo version on my examples page.