I've had great experience using Zepto, Backbone & custom styles. Was able to field a single-page app with about a dozen views - very interactive and very fast. Total app size about 25K including support libraries. Data loads asynchronously, cached using localstorage. I ran into endless glitches with JQM, but Zepto/Backbone/CustomCSS has been bulletproof every step of the way.
I'm heading this way on a PhoneGap app I'm working on right now. Once you give up the goal of a pixel-perfect emulation of a stock iOS UI the combination of Zepto/Backbone/CSS is pretty powerful and much, much faster than Cocoa for prototyping.