IE has an interesting behavior with regard to z-index.  When IE compares z-indices of two positioned elements (e.g., position:relative) that do not have explicit z-index declarations, the element appearing later in the DOM always wins.  And this applies to the contents of those positioned elements also.

The yui-ac class, which wraps the input field and the suggestion container in AutoComplete, is position:relative and has no z-index (see http://developer.yahoo.com/yui/examples/autocomplete/ac_skinning.html for a peek at the Sam Skin CSS for AutoComplete).  No matter what you do with the z-index of your suggestion container (.yui-skin-sam .yui-ac-container), it will still appear beneath a positioned element appearing later in the page.  In your case, part of the Sam Skin CSS for Button declares position:relative (for IE) for a portion of the button's contents.  That portion of the button's contents lays on top of the AC suggestion container.

By ensuring that the highest-level positioned ancestor of your AC container has an explicit z-index greater than zero, you should resolve your problem.  And it's likely that the .yui-skin-sam .yui-ac declaration is that highest-level parent.  (If the fix above doesn't work, look for a positioned element that is an ancestor of your AutoComplete containter and that does not have a z-index.)

Regards,

Eric