I'm using jakarta-poi-1.9.0-dev-20030122.
If I create a workbook containing formulas and then I try to call 
cell.getNumericCellValue on formula cells, returns 0.0
(the same happens if I save the workbook on disk and then I read it again with 
POI)
If I open such a workbook with Excel2000, it works, but when I try to close the 
document (without any manual change), Excel prompts me with a question: "Do you 
want to save the changes you made to POI.xls? Microsoft Excel recalculates 
formulas when opening files last saved by an earlier version of Excel".
Now, if I say "yes", and I read the newly saved workbook with POI, the 
cell.getNumericCellValue on formula cells returns the correct value.

Here I put a short Java code presenting the problem, then I will attach two 
very simple xls (POI.xls and Excel.xls), the first created with POI and 
containing a formula cell that is not calculated by POI when asked to, the 
latter is the Excel saved version of the same workbook (it is quite bigger than 
the original).

HSSFWorkbook wb = new HSSFWorkbook();
HSSFSheet sh = wb.createSheet("Sheet");
HSSFRow row = sh.createRow((short) 0);
HSSFCell cell = row.createCell((short) 0);
cell.setCellValue(3.18);
cell = row.createCell((short) 1);
cell.setCellFormula("A1");
//wb.write(new FileOutputStream("POI.xls"));
//wb = new HSSFWorkbook(new FileInputStream("POI.xls"));
//cell = wb.getSheet("Sheet").getRow(0).getCell((short) 1);
System.out.println(cell.getCellFormula());
System.out.println(cell.getNumericCellValue());

This prints out:

A1
0.0

The same happens if you uncomment the three commented lines

By the way, since I know that formulas are under development, today I wanted to 
download the last nightly release before submitting this bug, but I found the 
nightly directory is empty ...