the main problem with that is when certain topics with large objects like images or preformatted text which stretches the topic out. If the banner was within the same constricted table as the forum tables then you would lose your navigation menu off the screen because that table would be stretched out as well.
I suppose I worded that incorrectly what I meant was the left margin of the Title image. I am sure it is easily done if you know how just noticed it is set left of the remainder of the page slightly.