Gridview row editing - dynamic binding to a DropDownList

I'm trying to get an ASP.NET 3.5 GridView to show a selected value as string when being displayed, and to show a DropDownList to allow me to pick a value from a given list of options when being edited. Seems simple enough?

My gridview looks like this (simplified):

<asp:GridView ID="grvSecondaryLocations" runat="server" 
              DataKeyNames="ID" OnInit="grvSecondaryLocations_Init" 
              OnRowCommand="grvSecondaryLocations_RowCommand" 
              OnRowCancelingEdit="grvSecondaryLocations_RowCancelingEdit"
              OnRowDeleting="grvSecondaryLocations_RowDeleting"
              OnRowEditing="grvSecondaryLocations_RowEditing" 
              OnRowUpdating="grvSecondaryLocations_RowUpdating"  >
<Columns>
    <asp:TemplateField>
         <ItemTemplate>
              <asp:Label ID="lblPbxTypeCaption" runat="server" 
                                 Text='<%# Eval("PBXTypeCaptionValue") %>' />
         </ItemTemplate>
         <EditItemTemplate>
                      <asp:DropDownList ID="ddlPBXTypeNS" runat="server" 
                               Width="200px" 
                               DataTextField="CaptionValue" 
                               DataValueField="OID" />
         </EditItemTemplate>
    </asp:TemplateField>
</asp:GridView>

The grid gets displayed OK when not in editing mode - the selected PBX type shows its value in the asp:Label control. No surprise there.

I load the list of values for the DropDownList into a local member called _pbxTypes in the OnLoad event of the form. I verified this - it works, the values are there.

Now my challenge is: when the grid goes into editing mode for a particular row, I need to bind the list of PBX's stored in _pbxTypes.

Simple enough, I thought - just grab the drop down list object in the RowEditing event