We're having troubles with a certificate getting broken when a workbook is
saved.

Dean's PC: Running Excel 2007 in compatibility mode, we sign the VBA
project with a VeriSign certificate, and save the workbook as a .XLS file.
As long as we're in 2007 compatibility mode on this or other PCs, the
certificate behaves very nicely. We can save the file, re-open it and the
certificate works perfectly. [:~}

Old PC: We open that very-same signed .XLS workbook in Excel 2003, and the
certificate is respected (this time). We run no macros and make no edits.
We save the file and then close Excel. We re-open that workbook (still in
Excel 2003) and the certificate is broken -- doesn't match the code and
macros are disabled! [:~{

We then try opening that same workbook in 2007 compatibility mode, and
indeed the certificate is broken.