I managed to get a slightly longer gdb output, looks like it's stucked in some inner recursive event-loop.

(gdb) bt
#0  QWidget::event (this=0x84c440, event=0x7fffff7ffc00) at kernel/qwidget.cpp:8347
#1  0x00007ffff504d3c6 in QFrame::event (this=0x84c440, e=0x7fffff7ffc00) at widgets/qframe.cpp:557
#2  0x00007ffff51618ab in QAbstractItemView::viewportEvent (this=0x84c440, event=0x7fffff7ffc00) at itemviews/qabstractitemview.cpp:1644
#3  0x00007ffff51a94c5 in QTreeView::viewportEvent (this=0x84c440, event=0x7fffff7ffc00) at itemviews/qtreeview.cpp:1257
#4  0x00007ffff6009028 in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=<optimized out>, receiver=0x84c5f0, event=0x7fffff7ffc00)
    at kernel/qcoreapplication.cpp:986
#5  0x00007ffff4c3c85f in notify_helper (e=0x7fffff7ffc00, receiver=0x84c5f0, this=0x6af250) at kernel/qapplication.cpp:4555
#6  QApplicationPrivate::notify_helper (this=0x6af250, receiver=0x84c5f0, e=0x7fffff7ffc00) at kernel/qapplication.cpp:4531
#7  0x00007ffff4c420bf in QApplication::notify (this=<optimized out>, receiver=0x84c5f0, e=0x7fffff7ffc00) at kernel/qapplication.cpp:4102
#8  0x00007ffff6c8cb46 in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#9  0x00007ffff6008e9c in QCoreApplication::notifyInternal (this=0x7fffffffdd90, receiver=0x84c5f0, event=0x7fffff7ffc00) at kernel/qcoreapplication.cpp:876
#10 0x00007ffff4c3d862 in sendEvent (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#11 QApplicationPrivate::sendMouseEvent (receiver=0x84c5f0, event=0x7fffff7ffc00, alienWidget=0x84c5f0, nativeWidget=0x7fe120, buttonDown=0x0, 
    lastMouseReceiver=..., spontaneous=true) at kernel/qapplication.cpp:3170
#12 0x00007ffff4c3dcfa in QApplicationPrivate::sendSyntheticEnterLeave (this=<optimized out>, widget=0x819570) at kernel/qapplication.cpp:3254
#13 0x00007ffff4c8dff3 in QWidgetPrivate::hide_helper (this=0x8217c0) at kernel/qwidget.cpp:7635
#14 0x00007ffff4c8fd08 in QWidget::setVisible (this=0x819570, visible=<optimized out>) at kernel/qwidget.cpp:7801
#15 0x000000000042ec27 in ContactViewHoverButton::setVisible (this=0x819570, visible=false)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-view-hover-button.cpp:100
#16 0x0000000000425bce in QWidget::hide (this=0x819570) at /usr/include/qt4/QtGui/qwidget.h:498
#17 0x0000000000431041 in AbstractWidgetDelegateOverlay::hide (this=0x814440)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:164
#18 0x00000000004310b3 in AbstractWidgetDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:180
#19 0x0000000000431531 in HoverButtonDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:304
#20 0x0000000000432274 in HoverButtonDelegateOverlay::qt_static_metacall (_o=0x814440, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffff7fff90)
    at /home/mck182/Work/Telepathy/ktp-contact-list/build/contact-delegate-overlay.moc:244
#21 0x00007ffff601d281 in QMetaObject::activate (sender=0x84c440, m=<optimized out>, local_signal_index=<optimized out>, argv=0x7fffff7fff90)
    at kernel/qobject.cpp:3547
#22 0x00007ffff5163135 in QAbstractItemView::entered (this=<optimized out>, _t1=...) at .moc/release-shared/moc_qabstractitemview.cpp:369
#23 0x00007ffff5163298 in QAbstractItemViewPrivate::checkMouseMove (this=0x83db20, index=...) at itemviews/qabstractitemview.cpp:172
#24 0x00007ffff516ae0c in QAbstractItemView::mouseMoveEvent (this=0x84c440, event=0x7fffff800f60) at itemviews/qabstractitemview.cpp:1744
#25 0x0000000000423f98 in ContactListWidget::mouseMoveEvent (this=0x84c440, event=0x7fffff800f60)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-list-widget.cpp:587
#26 0x00007ffff4c8d178 in QWidget::event (this=0x84c440, event=0x7fffff800f60) at kernel/qwidget.cpp:8347
#27 0x00007ffff504d3c6 in QFrame::event (this=0x84c440, e=0x7fffff800f60) at widgets/qframe.cpp:557
#28 0x00007ffff51618ab in QAbstractItemView::viewportEvent (this=0x84c440, event=0x7fffff800f60) at itemviews/qabstractitemview.cpp:1644
#29 0x00007ffff51a94c5 in QTreeView::viewportEvent (this=0x84c440, event=0x7fffff800f60) at itemviews/qtreeview.cpp:1257
#30 0x00007ffff6009028 in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=<optimized out>, receiver=0x84c5f0, event=0x7fffff800f60)
    at kernel/qcoreapplication.cpp:986
#31 0x00007ffff4c3c85f in notify_helper (e=0x7fffff800f60, receiver=0x84c5f0, this=0x6af250) at kernel/qapplication.cpp:4555
#32 QApplicationPrivate::notify_helper (this=0x6af250, receiver=0x84c5f0, e=0x7fffff800f60) at kernel/qapplication.cpp:4531
#33 0x00007ffff4c420bf in QApplication::notify (this=<optimized out>, receiver=0x84c5f0, e=0x7fffff800f60) at kernel/qapplication.cpp:4102
#34 0x00007ffff6c8cb46 in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#35 0x00007ffff6008e9c in QCoreApplication::notifyInternal (this=0x7fffffffdd90, receiver=0x84c5f0, event=0x7fffff800f60) at kernel/qcoreapplication.cpp:876
#36 0x00007ffff4c3d862 in sendEvent (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#37 QApplicationPrivate::sendMouseEvent (receiver=0x84c5f0, event=0x7fffff800f60, alienWidget=0x84c5f0, nativeWidget=0x7fe120, buttonDown=0x0, 
    lastMouseReceiver=..., spontaneous=true) at kernel/qapplication.cpp:3170
#38 0x00007ffff4c3dcfa in QApplicationPrivate::sendSyntheticEnterLeave (this=<optimized out>, widget=0x819570) at kernel/qapplication.cpp:3254
#39 0x00007ffff4c8dff3 in QWidgetPrivate::hide_helper (this=0x8217c0) at kernel/qwidget.cpp:7635
#40 0x00007ffff4c8fd08 in QWidget::setVisible (this=0x819570, visible=<optimized out>) at kernel/qwidget.cpp:7801
#41 0x000000000042ec27 in ContactViewHoverButton::setVisible (this=0x819570, visible=false)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-view-hover-button.cpp:100
#42 0x0000000000425bce in QWidget::hide (this=0x819570) at /usr/include/qt4/QtGui/qwidget.h:498
#43 0x0000000000431041 in AbstractWidgetDelegateOverlay::hide (this=0x814440)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:164
#44 0x00000000004310b3 in AbstractWidgetDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:180
#45 0x0000000000431531 in HoverButtonDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:304
#46 0x0000000000432274 in HoverButtonDelegateOverlay::qt_static_metacall (_o=0x814440, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffff8012f0)
    at /home/mck182/Work/Telepathy/ktp-contact-list/build/contact-delegate-overlay.moc:244
#47 0x00007ffff601d281 in QMetaObject::activate (sender=0x84c440, m=<optimized out>, local_signal_index=<optimized out>, argv=0x7fffff8012f0)
    at kernel/qobject.cpp:3547
#48 0x00007ffff5163135 in QAbstractItemView::entered (this=<optimized out>, _t1=...) at .moc/release-shared/moc_qabstractitemview.cpp:369
#49 0x00007ffff5163298 in QAbstractItemViewPrivate::checkMouseMove (this=0x83db20, index=...) at itemviews/qabstractitemview.cpp:172
#50 0x00007ffff516ae0c in QAbstractItemView::mouseMoveEvent (this=0x84c440, event=0x7fffff8022c0) at itemviews/qabstractitemview.cpp:1744
#51 0x0000000000423f98 in ContactListWidget::mouseMoveEvent (this=0x84c440, event=0x7fffff8022c0)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-list-widget.cpp:587
#52 0x00007ffff4c8d178 in QWidget::event (this=0x84c440, event=0x7fffff8022c0) at kernel/qwidget.cpp:8347
#53 0x00007ffff504d3c6 in QFrame::event (this=0x84c440, e=0x7fffff8022c0) at widgets/qframe.cpp:557
#54 0x00007ffff51618ab in QAbstractItemView::viewportEvent (this=0x84c440, event=0x7fffff8022c0) at itemviews/qabstractitemview.cpp:1644
#55 0x00007ffff51a94c5 in QTreeView::viewportEvent (this=0x84c440, event=0x7fffff8022c0) at itemviews/qtreeview.cpp:1257
#56 0x00007ffff6009028 in QCoreApplicationPrivate::sendThroughObjectEventFilters (this=<optimized out>, receiver=0x84c5f0, event=0x7fffff8022c0)
    at kernel/qcoreapplication.cpp:986
#57 0x00007ffff4c3c85f in notify_helper (e=0x7fffff8022c0, receiver=0x84c5f0, this=0x6af250) at kernel/qapplication.cpp:4555
#58 QApplicationPrivate::notify_helper (this=0x6af250, receiver=0x84c5f0, e=0x7fffff8022c0) at kernel/qapplication.cpp:4531
#59 0x00007ffff4c420bf in QApplication::notify (this=<optimized out>, receiver=0x84c5f0, e=0x7fffff8022c0) at kernel/qapplication.cpp:4102
#60 0x00007ffff6c8cb46 in KApplication::notify(QObject*, QEvent*) () from /usr/lib/libkdeui.so.5
#61 0x00007ffff6008e9c in QCoreApplication::notifyInternal (this=0x7fffffffdd90, receiver=0x84c5f0, event=0x7fffff8022c0) at kernel/qcoreapplication.cpp:876
#62 0x00007ffff4c3d862 in sendEvent (event=<optimized out>, receiver=<optimized out>) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:231
#63 QApplicationPrivate::sendMouseEvent (receiver=0x84c5f0, event=0x7fffff8022c0, alienWidget=0x84c5f0, nativeWidget=0x7fe120, buttonDown=0x0, 
    lastMouseReceiver=..., spontaneous=true) at kernel/qapplication.cpp:3170
#64 0x00007ffff4c3dcfa in QApplicationPrivate::sendSyntheticEnterLeave (this=<optimized out>, widget=0x819570) at kernel/qapplication.cpp:3254
#65 0x00007ffff4c8dff3 in QWidgetPrivate::hide_helper (this=0x8217c0) at kernel/qwidget.cpp:7635
#66 0x00007ffff4c8fd08 in QWidget::setVisible (this=0x819570, visible=<optimized out>) at kernel/qwidget.cpp:7801
#67 0x000000000042ec27 in ContactViewHoverButton::setVisible (this=0x819570, visible=false)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-view-hover-button.cpp:100
#68 0x0000000000425bce in QWidget::hide (this=0x819570) at /usr/include/qt4/QtGui/qwidget.h:498
#69 0x0000000000431041 in AbstractWidgetDelegateOverlay::hide (this=0x814440)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:164
#70 0x00000000004310b3 in AbstractWidgetDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:180
#71 0x0000000000431531 in HoverButtonDelegateOverlay::slotEntered (this=0x814440, index=...)
    at /home/mck182/Work/Telepathy/ktp-contact-list/contact-delegate-overlay.cpp:304