Found the solution...  In the CSS file, control the background color of the list container with the following:

.igdd_DropDownListContainer
{
    background-color:White;
}

I'm having the same transparency issue, however setting the

EnableDropDownAsChild =false;

Does NOT resolve the problem. 

I'm running IE 9.02, and I'm dynamically creating and populating the WebDropDown.  Here is the block of code that creates and populates the dropdown:

if (fieldvaluelist != null && fieldvaluelist.Count > 0)

{

ScriptManager sm = new ScriptManager();

tc.Controls.Add(sm);

WebDropDown statuslist = new WebDropDown();

statuslist.ID = filtername;

statuslist.EnableMultipleSelection =true;

statuslist.MultipleSelectionType = DropDownMultipleSelectionType.Checkbox;

statuslist.BackColor = Color.White;

statuslist.DropDownContainerWidth = 250;

statuslist.EnableDropDownAsChild =false;

statuslist.EnableClosingDropDownOnSelect = false;

statuslist.DisplayMode = DropDownDisplayMode.DropDownList;

statuslist.ViewStateMode = System.Web.UI.ViewStateMode.Inherit;

statuslist.Visible = true;

foreach (XmlNode nd in fieldvaluelist)

{

string itemName = nd.Attributes["value"].Value;

string itemCode = nd.Attributes["key"].Value;

Infragistics.Web.UI.ListControls.DropDownItem ddi = new DropDownItem(itemName, itemCode);

statuslist.Items.Add(ddi);

}

//----------------------

Any ideas what I'm missing?  Any help is greatly appreciated.

For anyone who has this issue in the future.  The problem seems to be from placing an infragistic's webdropdown control on an element that is positioned using CSS and absolute positioning.  After googling this issue I found the solution.  Simply set

EnableDropDownAsChild="false" 


Dave.