I tripped over this link recently:
Its a good overview of some of the security features in Office 2007 and links to some MS resources.
For the avoidance of any doubt, worksheet, workbook, workbook open and VBA protection are not considered security features. These are usability features.
You’ll see from the documents at that link that MS have a much better security story by integrating more closely with Windows and policies etc.
As a matter of interest, please leave a comment if you care about security (honestly). If you don’t care about security please leave a comment why.
I think a great many people in the Office development world don’t really care about security, so don’t be shy.