I have been struggling with fixed positioning in iPad for a while. I know iScroll and it does not always seem to work (even in their demo). I also know that Sencha has a fix for that, but I couldn't ctrl+f the source code for that fix.
I am hoping that someone may have the solution. The problem is that fixed positioned elements do not get updated when the user pans down/up on an iOS powered mobile Safari.
I ended up using the new jQuery Mobile v1.1: http://jquerymobile.com/blog/2012/04/13/announcing-jquery-mobile-1-1-0/
We now have a solid re-write that provides true fixed toolbars on the a lot of popular platforms and safely falls back to static toolbar positioning in other browsers.
The coolest part about this approach is that, unlike JS-based solutions that impose the unnatural scrolling physics across all platforms, our scrolling feels 100% native because it is. This means that scrolling feels right everywhere and works with touch, mousewheel and keyboard user input. As a bonus, our CSS-based solution is super lightweight and doesn’t impact compatibility or accessibility.