I have Office 2010 x64 running on Windows 7 x64.

Open a document, it first opens in 'Protected mode' in office. You can click 'Enable editing',
At this point, the formatting is completely skewed. It goes from a column spreadsheet with 100s of rows, to a 3 column spreadsheet with thousands of rows

Correction from above : when you click "Endable editing" it loads a version of the Confluence Login page into your browser worksheet


See attached document for screen shots

I found a workaround though – if save to the computer first, then open, click enable editing, it works fine.