I just ran into a problem last week with trying to use a fixed background within a div.
It looked great under IE, but was broken under Mozilla and Netscape.
When you change a background-attachment to fixed, it attaches it to the object under IE, but attaches it to the document for all other browsers it seems. Even IE for Mac attaches it to the document and not the object.
It makes more sense to attach it to the object as all you have to do is attach it to the body if you really wanted it there.
If you try to move the background to the center of your object, it moves out of alignment under IE.
The solution I found was to create a div and set it to the exact measurements of the background but not to set it as fixed. Then create a child element with overflow set to auto so it could scroll over the background.