I have found quite a few things.
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".