I think it has something to do with the "body" class in the skin css.

What I have seen proposed as the fix, but not yet tried myself, is to use "#body" instead of "body"

maybe one of the skinning gurus can explain it...

**edit

I extracted the "body" class from my skinn.css file, used the admin > site settings > stylesheet editor
and replaced it's "body" class with the skin.css body class, and added the "#" in front of it. Now, it looks like this:

#BODY {
    background-image: url(/Portals/0/Skins/blahblahblah);
    MARGIN: 0px;
    background-color: #325872;}

Works perfectly in IE and FF- now the ftb editor has the expected white background.