Very nice. I noticed this a few days ago. Have you ever hit a website that contained scrollable content within a frame (box) and you couldn't scroll down or sideways wihin the frame using Safari? The content would just basically be cut off and static. With iOS 5 now that content is independently scrollable within the frame (box).
example: (scroll within the step1 and step2 boxes)
http://www.dynamicdrive.com/dynamicindex17/iframescroll.htm
example: (scroll within the step1 and step2 boxes)
http://www.dynamicdrive.com/dynamicindex17/iframescroll.htm