PhoneGap is a library for building out mobile phone applications. It does it by giving you a browser embedded in an application and letting you build the application using HTML and JS. There's lots of arguments for and against tools like PhoneGap (other libraries are available). But the one I keep arguing is something Zach said to me at DjangoSki.
Simply put, building apps with PhoneGap allows you to re-use existing skill sets without having to learn new stuff.
Many years ago at Enfold we were building an installer. We started using Inno installer which is a fine little installer and can be extended using Pascal. That was great fun to do, for a while. As the installer grew and took on more features, the ability to interact more with Windows got more complicated. The installer is quite complicated and requires configuring of IIS and interaction with Active Directory, fun stuff indeed. At some point we had to stop trying to build all this in Pascal and move to Python.
So the installer moved to a two step process, Inno installs Python and other dependencies and spats. That's a little process that fires up a local window with a browser window embedded pointing to a Python process, running a simple HTTP that serves back HTML from Page Templates. The user interacts with a browser that looks reasonably native and we process and do all the remaining work in Python.
The result was using Python and HTML allowed us to have a product that was easily maintained and updated by anyone developer in the company.
Using the same technologies for building websites, mobile applications and desktop applications is incredibly helpful. You get to use the same tool chain throughout. That means you can move developers between projects quickly and tools that developers are already familiar with can be used. That makes your developers far more productive.
Keep learning, the right bits
I didn't have any problem with re-learning Pascal for the installer. It was quite good fun. Learning stuff happens all day, every day, it's an essential part of our careers. However I think it's important to learn and move forward on the right bits. Time and brain space is limited and there's an almost limitless amount of things that can be learnt. Focusing on things that can give the maximum return is crucial.
So that's why I think PhoneGap is useful, not the cross platform stuff, but because it let's you re-use existing resources in your organization. It allows you to focus time on learning new ways to be awesome within your current realm. Or as Zach said over a beer (paraphrasing) "I could get our web developers up and running on it in no time".