Hello David,

You can handle the CellExporting on the excelExporter and create comment and then attach it to the current exported cell:

  protected void UltraWebGridExcelExporter1_CellExporting(object sender, Infragistics.WebUI.UltraWebGrid.ExcelExport.CellExportingEventArgs e)
    {
            if (e.Value != null)
            {
             WorksheetCellComment mycomment = new WorksheetCellComment();
             mycomment.Text = new FormattedString(e.Value.ToString());
             e.CurrentWorksheet.Rows[e.CurrentRowIndex].Cells[e.CurrentColumnIndex].Comment = mycomment;
            }   
}

I hope this helps.

Sincerely,
Dimi
Developer Support Engineer
Infragistics, Inc.