Same issue for OOo 3.0.0 (OOO300m9) running on Linux.

When you save a workbook, zeros values are not saved if they are formula results.

This is not a problem when "Tools / Cell Contents / AutoCalculate" is since,
when oppening the workbook, formulas are recalculated and their results are shown.

However, if "AutoCalculate" is off recalculation doesn't happen and zeros are
left blank (even for "Tools / Options / OpenOffice.org Calc / View / Zero
values" checked).

In attachment an example workbook. A zero appears on A1 because it's not a
formula. Zeros should appear in cells A2, A3, A4 and A5 but they don't. Press F9
to force calculation and you will see all zero values.

I hacked the workbook file to get more information.

In content.xml we see that if a cell contains a formula whose result is zero,
then 'table:table-cell' element doesn't have 'office:value' and
'office:value-type' attributes and its 'text:p' element is empty.

I've manually included 'office:value' and 'office:value-type' (assigned to
"float") and 'text:p' (containning 0) but it doesn't help unless 'office:value'
is different from 0 (but in this case we see a wrong value when we open the
workbook).

I guess this is both an open-import and a save-export issue.