This sample code does not produce the expected result (value is present in the 
cell but not visible).

import java.io.*;
import org.apache.poi.hssf.usermodel.*;

public class Test {

   public Test() {
   }

   public static void main(String[] args) {
      try {
         FileOutputStream out = new FileOutputStream("test.xls");

         HSSFWorkbook workbook = new HSSFWorkbook();
         HSSFSheet sheet = workbook.createSheet("Test");
         HSSFRow row = sheet.createRow(0);
         HSSFCell cell = row.createCell((short)0);
         HSSFDataFormat fmt = workbook.createDataFormat();
         HSSFCellStyle style = workbook.createCellStyle();
         style.setDataFormat(fmt.getFormat("$#,##0"));
         cell.setCellStyle(style);
         cell.setCellValue(1234.56);
         workbook.write(out);

         out.close();
      }
      catch (Throwable t) {
         t.printStackTrace();
      }
   }
}