Fluid layouts are a reasonable compromise, but they mean a layout that's slightly "off" in many cases. Look at how GTK+ scales on different mobile devices, for instance.
It works, but it means giving up a lot of control, and the results tend to show it.
forgetting the fact that GTK is quite some years old and software has evolved. You really think it's impossible to have a scalable UI? Ever surfed the internet with different resolutions? Now imagine you don't have cross browser incompatibilities and a framework that behaves the same everywhere.
When it comes to other features of the phone, e.g. camera or accelerometer, like one of the posters said- you'll have to do progressive enhancement. Intelligent web developers already know how to do this. You feature test, then you enable a particular feature. Its not all that difficult.