If this is happening with JQM 1.2, you should carefully document it and file an Issue on Github, once you are sure it is not due to something you are doing. (Which is impossible for us to help you with based on just a picture.)
If you are using an older version of JQM, be advised that the JQM developers are not mind-readers, and older versions were released prior to the release of iPhone 5.
Posting something more than a picture would be helpful.
Is your page actually long enough that it's longer than the viewport display? The hiding of the URL address bar works by performing a scroll operation on the page. If your page is actually shorter than the browser viewport I don't believe the URL bar will be hidden in these cases because the page isn't long enough to scroll.