I have a site that's done using ASP.NET MVC and jQuery. Is it possible to modify my existing project without too much of rework so that it can be used in phonegap to create iphone/android apps?
Here's an approach: move your logic to an MVC WebAPI (or other REST/webservice) project, then convert the MVC site into a simple html/javascript/css/image site (Mobile site). Then refactor your Mobile site to use Ajax/JS to query the WebAPI/Rest services you created. Once you've separated your code this way, you can then package the Mobile site with Phonegap. I'm not sure how much work that will be for you or your project. If you're using a lot of Html Helpers or Razor markup in your views it may be too involved.
The core point of my suggestion here is to separate your mobile UI layer and the backend processing layer so you can only package the Html5/UI/Javascript layer with Phonegap and leave the backend processing on your web server. I don't think I need to explain this, but obviously the app packaged with Phonegap is not going to have the MVC/.Net framework available on the mobile device to render views or execute controllers, etc. By migrating your UI to be simple Html5 and Javascript you can use Ajax/Jquery/Javascript calls against your backend, which you will probably want to host in ASP.Net MVC WebAPI.
Edit: Guess there was some confusion about my suggestion. I'm not saying this is the only way to do go, but this is what I'm familiar with as it's how our team builds our desktop/web + mobile + phonegap + mvc4 + webapi + kendoui application. This pattern works well for us so maybe it'll work for you too, or at least give you some ideas on how to structure your solution. Good luck!