Good question. As Seb pointed out we do have quite a bit of coverage around the PhoneGap topic, and in Chapter 12 we do make use of an excellent PhoneGap plugin by Boris Smus to integrate with Android intents inside a web app.
With the book having a focus on the web side of mobile application development I decided not to throw people into the deep end with writing any Java code, but the process is reasonably easy to understand and I would be more than happy to provide some information on the topic online somewhere if this is something you can't find elsewhere. I think Boris' post provides some good information on the process though and is an excellent starting point.
I think the guys at Nitobi (creators and maintainers of PhoneGap) will provide improved documentation on how to write PhoneGap plugins in time, but for now if you have happy reading some source code, then there is a repository on GitHub that has some pre-built plugins that you can have a look at.
If I have to use a middleware such as PhoneGap, that means I have to develop an app on the Android phone, install it (post it in the Android market if I want other users to be able to access my web-based pages), and only then shall I be able to use web-page-based apps taking advantage of the webcam interface...
Or am I missing something ?