The tolerance-options 'fit', 'intersect' and 'touch' don't work as expected in the demo on http://ui.jquery.com/1.5a/demos/droppable.html : This seems due to the margin of the draggables. As an example I will describe the 'touch'-option:

If the draggable has "margin: 0" everything is fine. As soon as the rightmost pixels of the draggable touch the leftmost pixels of the droppable, the droppable gets activated.

However if there is a "margin: 10px" specified on the draggable (like in the demo) the droppable as activated when the draggable's right edge overlaps the droppables left edge by exactly 10px.

Adding a margin to the droppable however doesn't brake things.

I've tested this issue using Firefox 2 and 3. In Opera the droppable-demo doesn't work at all.