I'm noticing this issue. I made a quick screen capture to demonstrate: http://dl.dropbox.com/u/904456/2010-06-14_2323.swf
Basically when you have a min-width set and the viewport goes under that constraint, a horizontal scroll bar appears. Pretty much what you would expect, but when you scroll over horizontally all elements that are suppose to extend across the entire width of the page and have background images/colors different from the body do not extend. If you resize the viewport it seems to catch up.
Is this a known issue? You can see it on a lot of sites, http://gowalla.com for example.
Any ideas?