Well, they say it's done by taking "advantage of native scrolling":
http://jquerymobile.com/test/docs/toolbars/bars-fixed.html
And there obviously is improvement over the previous version, in that, earlier there was some flickering of fixed elements on desktop view-port.
I stand corrected, and appreciate the effort jQuery team.
http://cubiq.org/iscroll-4
And, as you say, use the 'fixed' CSS in newer Safari versions that support it.