Thank you for your bug report. That file is available
in the package qt3-apps-dev and is installed in /usr/include/qt3/qassistantclient.h
This is in both Breezy and Dapper.
** Changed in: qt-x11-free libqt3-mt-dev (Ubuntu)
Status: Unconfirmed => Rejected
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qassistantclient.h missing
https://launchpad.net/malone/bugs/26100