I *think* Web Runtime will be only opt-in in Maemo 5: you will get it only if you install something using it.

In Maemo 6 it will be pre-installed as official runtime, next to Qt that will be the official native environment. All the rest (including GTK+) will come only when users install apps depending on them.

There is a timeline at http://www.slideshare.net/santtuahon...ia-web-runtime (linked in the first post).