I have a VF page that runs in an iFrame, it basically sits on top of the current page and allows the user to drill down to a specific product via a series of select lists.  The code is pretty straightforward:

    <apex:form id="addproductform">
        <apex:pageblock >
            <apex:messages styleClass="pbError"/>
            <apex:pageblockbuttons location="bottom">
                <apex:commandbutton value="Save" action="{!Save}" />
                <apex:commandbutton value="Cancel" action="{!Cancel}" />
            </apex:pageblockbuttons>
            <apex:panelgrid columns="6">
                <apex:outputlabel value="Operating Company"/>
                <apex:outputlabel value="Division"/>
                <apex:outputlabel value="Segment"/>
                <apex:outputlabel value="Job Code"/>
                <apex:outputlabel value="Competency"/>
                <apex:outputlabel value="Skill"/>
                <apex:selectList id="OpCo_field" value="{!OpCo}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!OpCompaniesList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeOpCo}" rerender="addproductform" />
                </apex:selectList>
                <apex:selectList id="Division_field" value="{!Division}" disabled="{! If(OpCo='',true,false)}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!DivisionsList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeDiv}" rerender="addproductform" />
                </apex:selectList>
                <apex:selectList id="Segment_field" value="{!Segment}" disabled="{! If(Division='',true,false)}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!SegmentsList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeSegment}" rerender="addproductform" />
                </apex:selectList>
                <apex:selectList id="JobCode_field" value="{!JobCode}" disabled="{! If(Segment='',true,false)}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!JobCodesList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeJobCode}" rerender="addproductform" />
                </apex:selectList>
                <apex:selectList id="Competency_field" value="{!Competency}" disabled="{! If(JobCode='',true,false)}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!CompetenciesList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeCompetency}" rerender="addproductform" />
                </apex:selectList>
                <apex:selectList id="Skill_field" value="{!Skill}" disabled="{! If(Competency='',true,false)}" size="1" style="font-size:8pt;width:150px;">
                    <apex:selectOptions value="{!SkillsList}" />
                    <apex:actionSupport event="onchange" action="{!ChangeSkill}" rerender="addproductform" />
                </apex:selectList>
            </apex:panelgrid>
        </apex:pageblock>
    </apex:form>

It works like a champ in Firefox.  But when I load it in Internet Explorer, I don't get any options after the first list, and an error flag appears in the status bar.  The error IE is throwing is as follows:

Message: Unknown runtime error
Line: 123
Char: 58
Code: 0
URI: https://c.cs4.visual.force.com/faces/a4j/g/3_3_3.Finalorg.ajax4jsf.javascript.AjaxScript

Looking at the script, the code in question that's throwing the error appears to be this:

oldnode.outerHTML=new XMLSerializer().serializeToString(newnode);

Does anyone have any ideas on what could be causing this?  My customer needs a demo by Wednesday, and it's a little late to go back to the drawing board.