Disclaimer: The site has meanwhile been updated, this issue does not occur anymore with the new version of the design.
On a new site I built I noti
-webkit-overflow-scrolling:touch is the problem. If you use that anywhere in your CSS, the scroll-to-top shortcut stops working. This is a known problem with iOS 5, there is no workaround unfortunately. Hopefully it will be fixed in iOS 6.
You can have something similar to firebug buy using Safari and specifying your web-agent as iPhone. But I'm not sure you will have the same webkit engine.
You can also activate the Developers Tools for Safari in Settings > Safari > Advanced.
Good luck.