If you want to download jQuery Mobile source code and look into it just like everybody else, we are all out of luck :( As I mentioned in my previous post jQuery mobile facts, the source code will be available in October this year.
The jQuery Mobile source will not be in a separate jquery.mobile.js file. It will be right in the jQuery core. This means that jQuery team is fixing and improving jQuery core so that it works nicely in all major mobile phones and devices.
By the way, if you want to keep track of the jQuery mobile source code and be the first one to download it, when it is available, you should watch jQuery on GitHub.
The big part of the upcoming jQuery mobile is new UI components that work and render nicely in all mobile devices and degrade gracefully. So, keep an eye on jQuery UI as well. Here is the jQuery UI GitHub page.
Meanwhile you can read all jQuery mobile facts here.