Well, first off, none of this really has anything to do with jQuery Mobile. JQM can be useful for providing the mobile UI, though.
JSON is simply a standard format for packaging data for transmission. It serves a similar role as XML, but is simpler and can be easier to implement. You do not necessarily need to use JSON to develop your app.
I assume you do not want to put static web pages in your app for products, which would require that you release an update to the app every time you update your products or prices. So, you will need to get some/most/all data from your server.
The simplest thing would be to display pages directly from your server. You would need to make sure your server formats the pages appropriately for mobile, of course. You might do this for catalog pages, ordering, or both.
You might want to cache content on the app so that the user doesn't need to be online to use the app.
Alternately, you can format the pages in the app, after retrieving only the needed data (not formatted HTML pages) from your server. JSON would be a suitable format for that. You would need to add a "web service" to your server which would be used by your app to retrieve the data. Your app might retrieve data when needed, and cache it for later use. Or, you might have the app "sync" at startup, and retrieve any updates, keeping an internal database up-to-date.
I don't know if I would bother with this, though, because it is difficult to get approval for the Apple App Store for an app that simply duplicates the functionality of a website, and because the level of effort for a small store probably exceeds the potential benefit. Why not just use JQM to create a mobile-oriented website? Later, if you feel the need, you can re-purpose much of your effort to create a webapp and/or native app.
I am sure you can find plenty of samples, but I suspect that looking around on the PhoneGap site and forums and PhoneGap-related searches will be more fruitful than asking here.
There are a number of alternatives to PhoneGap that you might consider, including Titanium and (my favorite) Rhodes.
PhoneGap simply packages a static site inside of a native app that uses a native webview control to display the content. It provides some Javascript extensions that permit access to device resources using Javascript. You will have to do all your programming in your PhoneGap app in Javascript. (Unless you write your own extensions, which have to be done in the native language of the device.)
Rhodes takes a much different approach, and runs a Ruby server within the native app. So, you can write database and controller code in Ruby, and the HTML/CSS/Javascript gets it's pages from the server. If you are a Rails developer, then Rhodes will provide a very familiar environment. (Although it is similar, though, Rhodes does not attempt to be replicate or be compatible with Rails.)
As far as I know, though, nobody has taken a similar approach to embed a PHP server that would allow you to leverage your PHP skills.
