There is a standard 'number' format for a date in a cell in Excel of "dd-mmm-
yy", with the month in three letters. This is not one of the standard formats 
offered by OOo.

In Excel, the "dd-mmm-yy" format is listed in the selection menu for 'Format-
Cell-Number-Date'.
The "dd-mmm-yy" format is not listed in this menu in OOo (SO6, SO7, OOo1.0.x, 
OOo1.1.0).

The "dd-mmm-yy" format is imported from an Excel file as "d mmm yy" (which is a 
standard OOo date format), dropping the leading 0 from the day number and 
losing the hyphens.

If the date format is set as "dd-mmm-yy" in an OOo document and exported 
to '.xls' file format, and then reopened with OOo, there is no problem, but if 
you open and save the file with Excel, and then reopen the file with OOo the 
cell format becomes "d mmm yy" in OOo, although it will remain "dd-mmm-yy" if 
the same file is opened in Excel.  It's the import action in the OOo filter 
that causes the problem.

The situation has been found with import of files edited with:
Excel95 (W95), Excel97 (W95, W98), Excel2000 (W98SE, W2000, XP), ExcelXP (XP) 
[all UK]; Excel97 (W98SE), ExcelXP (XP) [Canada/US]. {NB: The Excel 97 has 
included both SR-1 and SR-2 versions.}

and with import to:
SO6 [with at various times, PP1, PP2 & PP3] (XP, W98, W98SE, Linux-SuSe8.0, 
Linux-SuSe8.2), SO7 (XP), OOo1.0.1 (Linux-Debian), OOo1.0.3 (Linux-Debian), 
OOo1.1.0 (XP, Linux-Debian, Linux-SuSe8.2). {The Debian Linux's are permanent 
installs from a Knoppix 3.1 CD, kernel 2.4.20.}

Same date format import situation everywhere.

Latest test procedure:
Saved a spreadsheet from OOo1.1.0 (XP) to '.xls' file with number format in a 
date cell of "dd-mmm-yy".
Reopen file in OOo - no change, cell displays as "dd-mmm-yy".
Open file in OOo (Linux-Knoppix3.1) - no change, cell displays as "dd-mmm-yy".
Open file in Excel 97 (SR-1) - no change, cell displays as "dd-mmm-yy".
Save file under new name in Excel 97 (SR-1) - no change, cell displays as "dd-
mmm-yy".
Reopen both files in Excel 97 (SR-1) - no changes, both display as "dd-mmm-yy".
Reopen both files in OOo (XP and Linux) - original file displays as "dd-mmm-
yy", new file saved from Excel displays as "d mmm yy".

What I think may be happening is that Excel saves the actual "dd-mmm-yy" format 
as a default code, and the filter translation into OOo outputs this default 
code as "d mmm yy".