I have an HTML page with a fixed-height div which should be scrollable (only vertically). In iOS 5 this can be achieved using:
overflow-y: a
I had exactly the same issue. The problem turned out to be caused by two zero size iframes my site used to track history changes and load scripts. Removing these fixed the issue. I filed a bug with apple, waiting to hear back from them.
Check to see if you have any iframes on your page they could be the cause.