I am currently starting to build a mobile application using jQuery Mobile and wrap it with Phonegap for native app support, it is an extension to our already existing desktop ap
One of the things you will need to look out for is same-origin features built into jquery. Since phonegap uses a a webview withl url file:// any web request you make will be cross domain so you need to configure jquery mobile to allow it.
see http://jquerymobile.com/test/docs/pages/phonegap.html
Other than that, there's not much difference between a phonegap mobile application and a regular web app with respect to getting/posting JSON.