If I develop a web app in phonegap, can the same web app be made to run in regular desktops/laptops inside a browser?
As Remy said, most of the standard stuff you write will be fine, but you'll need to wrap certain methods that use the camera/accelerometer to see if you're in an app or not.
Athough a better solution is to use feature detection, as you can upload pictures and use the camera with the HTML input in a mobile website, I'm pretty sure you can access the web cam in certain versions of Desktop Chrome now.
If you want to build a properly universal app, you'll need to use lots of feature/device detection, or have a central core product, and use something like Git Submodules to include and extend it as a Phonegap app.
There is no single, easy, standardised way of doing it.