The KDE API Reference (api.kde.org) web site is now providing ".qch" files suitable for loading into Nokia's Qt Assistant.
Look for the "[qt]" links.
Currently, auto-regeneration of these files happens on a nightly basis for kdelibs and kdepimlibs trunk, and for kdelibs and kdepimlibs 4.5. More can be added, upon request.
To use load these files into assistant:
I'll leave it to the experts to tell us how best to load these files into KDevelop or QtCreator.
Warning: these files are not small.