You could also do top left instead of 0%. The shorthand version,
background: transparent url(../images/main_top.png) top left repeat-x;
Or sub color for "transparent."
This may be one of those cases where the div actually has no height and the float caused a "shrink wrap". Add this right after your text before closing .Border_top:
and I'd bet it will do the same thing. You can also put a border on the div just to see what's happening.