I have recently written a sticky module as part of a redesign of a site using Semantic.
I have also written a visibility API for triggering events on element visibility
These will both be part of release version of SUI, but I have not had the time to write documentation for yet.
Visibility
Their usage is like $('.whatever').visibility('top visible', function() { console.log('do something'); } );
For more available functions you'll have to look directly at JS, but includes things like "top passed" "bottom passed" "bottom visible" "passed 50%" etc.
Sticky
This is used specifically for menus that hit horizontally next to content and stay fixed while this content passes. This could be used, for example, for menus like the Semantic UI definition docs following menu.
Usage is like $('.ui.sticky').sticky({ context: '.element'}); where context is the element which it will be bound by. This might be difficult to use without some examples so keep that in mind. But I will get them out there shortly.