There are no less than 3 views on the file.
2. The bigger view to the right of the explorer file pane.
3. The right click on the file and selecting Preview.
4. Right click and Open the file in Excel.
From memory item 1 does not require a password, item 2 may be true as well but 3 and 4 should require the password.
I think folk thought this was a bug. Call MSFT or post in their forums if you want to get it straight from the horse's mouth.
You could "make my files private" in the meantime.
Just updgraded to Windows 7. I have a few password protected Excel sheets. I was really surprised to see the content of my sheets showing in the Windows 7 Explorer preview pane (before opening the dicument and being asked to enter the password). Any extra protection I need to put on the document to prevent this behavior?