I use the QtCurve and have set it to be used for GTK applications too, in System Settings -> Applications appearance -> GTK+ appearance (I'm translating from portuguese, english labels may not be exactly these).
Before upgrading to KDE 4.9.1, which is available in Ubuntu 12.04 backports repo, GTK applications appearance was very close to the KDE ones, both using QtCurve.
However, after upgrading to 4.9.1 and QtCurve 1.8.14-1, all GTK applications show the default appearance of QtCurve, which is an oxygen-like style.
I use QtCurve with a customized acqua-like style which is correctly rendered for KDE applications, but not for GTK applications. Therefore, GTK applications look completely alien in my KDE desktop. The same thing happens if I set up QtCurve to use any of the provided preset settings. Only if QtCurve is set up to use the default oxygen-like look and feel do KDE and GTK applications look the same.
Running Kubuntu 12.04.01 AMD64 with all updates available to this date (16-sep-2012).
QtCurve packages (all versioned 1.8.14-1~precise1~ppa1):
qtcurve
kwin-style-qtcurve
kde-style-qtcurve
qtcurve-i18n
gtk2-engines-qtcurve (this one is versioned 1.8.14-1)
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gtk2-engines-qtcurve 1.8.14-1
ProcVersionSignature: Ubuntu 3.2.0-30.48-generic 3.2.27
Uname: Linux 3.2.0-30-generic x86_64
ApportVersion: 2.0.1-0ubuntu13
Architecture: amd64
Date: Sun Sep 16 10:53:07 2012
InstallationMedia: Kubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120423)
ProcEnviron:
LANGUAGE=pt_BR
TERM=xterm
PATH=(custom, user)
LANG=pt_BR.UTF-8
SHELL=/bin/bash
SourcePackage: gtk2-engines-qtcurve
UpgradeStatus: No upgrade log present (probably fresh install)
Status changed to 'Confirmed' because the bug affects multiple users.