For anyone that might not be familiar with the issue yet, Office 12 introduces a rather large number of new file formats.

There are formats that include macros (DOCM, XLSM, etc), formats that explicitly exclude macros (DOCX, XLSX, PPTX), and, for the Excel aficionados out there, a whole new binary format, XLSB.

This new binary format is not the old style XLS format, although both are binary and relatively proprietary. It is, however, based on a ZIP container file, just like the other new Office formats. This is a big difference from the older DOC and XLS files, which were OLE Structured storage files.

Stephane Rodriguez has written an excellent article on Code Project about the new format, with some good info on the old formats too.

Very interesting that the BIN files within the zip container turn out to be OLE Structure Storages almost exactly like what the old formats used, just wrapped up in a ZIP container instead of a Stuctured Storage container.

So much for really simplifying anything.