Damon and/or Sébastien: In your experience developing Android web apps, have you ever encountered cross-platform issues, either as a result of different API levels or different builds of WebKit?
There are some minor difference between the webkit engine depending the API version you are using (1.5, 2.1, 2.2) but the code shown is the book is cross-api compatible, there some exceptions but which are explained in notes.
In the book itself, Chapter 7 works through some samples using the HTML5 Canvas and touch events, and both of these are those will probably cause you the most pain as you strive for a cross-platform (which includes dealing with the differences within Android itself) mobile web app solution. In many ways though, you are still better off going this way than developing a purely native application as with the web approach you end up having to tailor and tweak 20% of your code for a specific platform, whereas a native app would probably require a ground up build to target different devices.
I have to say I understand what Joe is saying when he makes this comment in his closing remarks (with reference to a bug that is present in the Android 2.3 SDK):
This is a REALLY bad thing, since it makes web development on Android even more frustrating, and will chase developers away from that platform.
I genuinely hope that this doesn't happen, but it's a genuine risk. The good news is that if you target Android first in your mobile web app development process, then the other platforms actually almost "just work".