This could very well be a page life-cycle problem - it really depends on where you execute this code. Please, use this code in the initialize event of the grid. Also, could you please try using fromData.ID instead of UniqueID?

I just went through all these steps and it worked great for me. Here is my setup and code:

protected void UltraWebGrid1_InitializeLayout(object sender, LayoutEventArgs e)
    {
        UltraGridColumn GridColumn = new UltraGridColumn();
        GridColumn.BaseColumnName = "BirthDate";
        GridColumn.Header.Caption = "Date";
        GridColumn.Header.RowLayoutColumnInfo.OriginY = 0;
        GridColumn.Width = Unit.Percentage(25);
        GridColumn.Key = "BirthDate";
        UltraWebGrid1.Bands[0].Columns.Add(GridColumn);
        GridColumn.AllowUpdate = AllowUpdate.Yes;
        GridColumn.Type = ColumnType.Custom;
        GridColumn.FieldLen = 10;
        GridColumn.HTMLEncodeContent = true;       
        GridColumn.DataType = "System.DateTime";

        GridColumn.EditorControlID = WebDateChooser1.ID;
        WebDateChooser1.DropDownStyle.BackColor = System.Drawing.Color.Blue;       
        WebDateChooser1.Format = Infragistics.WebUI.WebSchedule.DateFormat.Short;       
    }

<igtbl:UltraWebGrid ID="UltraWebGrid1" runat="server"
        DataSourceID="AccessDataSource1" Height="200px" Width="325px"        
        oninitializelayout="UltraWebGrid1_InitializeLayout">
        <Bands>
            <igtbl:UltraGridBand>
                <Columns>
                    <igtbl:UltraGridColumn BaseColumnName="FirstName" IsBound="True"
                        Key="FirstName">
                        <Header Caption="FirstName">
                        </Header>
                    </igtbl:UltraGridColumn>
                    <igtbl:UltraGridColumn BaseColumnName="LastName" IsBound="True" Key="LastName">
                        <Header Caption="LastName">
                            <RowLayoutColumnInfo OriginX="1" />
                        </Header>
                        <Footer>
                            <RowLayoutColumnInfo OriginX="1" />
                        </Footer>                       
                    </igtbl:UltraGridColumn>
                    <igtbl:UltraGridColumn BaseColumnName="EmployeeID" DataType="System.Int32"
                        IsBound="True" Key="EmployeeID">
                        <Header Caption="EmployeeID">
                            <RowLayoutColumnInfo OriginX="2" />
                        </Header>
                        <Footer>
                            <RowLayoutColumnInfo OriginX="2" />
                        </Footer>
                    </igtbl:UltraGridColumn>
                </Columns>
                <AddNewRow View="NotSet" Visible="NotSet">
                </AddNewRow>
            </igtbl:UltraGridBand>
        </Bands>
        <DisplayLayout BorderCollapseDefault="Separate" Name="UltraWebGrid1"  AllowUpdateDefault="RowTemplateOnly"
            RowHeightDefault="20px" Version="4.00">           
            <FrameStyle BorderStyle="Solid" BorderWidth="1px" Font-Names="Verdana"
                Font-Size="8pt" Height="200px" Width="325px">
            </FrameStyle>
            <Pager>
                <PagerStyle BackColor="LightGray" BorderStyle="Solid" BorderWidth="1px">
                <BorderDetails ColorLeft="White" ColorTop="White" />
                </PagerStyle>
            </Pager>
            <EditCellStyleDefault BorderStyle="None" BorderWidth="0px">
            </EditCellStyleDefault>
            <HeaderStyleDefault BackColor="LightGray" BorderStyle="Solid">
                <BorderDetails ColorLeft="White" ColorTop="White" />
            </HeaderStyleDefault>
            <RowStyleDefault BackColor="White" BorderColor="Gray" BorderStyle="Solid"
                BorderWidth="1px" Font-Names="Verdana" Font-Size="8pt">
                <Padding Left="3px" />
                <BorderDetails ColorLeft="White" ColorTop="White" />
            </RowStyleDefault>
            <AddNewBox>
                <BoxStyle BackColor="LightGray" BorderStyle="Solid" BorderWidth="1px">
                    <BorderDetails ColorLeft="White" ColorTop="White" />
                </BoxStyle>
            </AddNewBox>
            <ActivationObject BorderColor="" BorderWidth="">
            </ActivationObject>
        </DisplayLayout>
    </igtbl:UltraWebGrid>
    <asp:AccessDataSource ID="AccessDataSource1" runat="server"
        DataFile="~/App_Data/Nwind.mdb"        
        SelectCommand="SELECT [FirstName], [LastName], [EmployeeID], [BirthDate] FROM [Employees]">
    </asp:AccessDataSource>
    <igsch:WebDateChooser ID="WebDateChooser1" runat="server">
    </igsch:WebDateChooser>