You can't set a Flash movie as the background of anything in HTML, such
as you might with a GIF or JPG. By using the wmode attribute, you can cause
SWF's to appear behind content whose CSS z-index is set higher than that of
the SWF's (which it inherits from the <div> -- or other HTML tag -- it's
in). Search for wmode on the Macromedia site and you'll get some good
insight.
David
stiller ( at ) quip ( dot ) net