valgrind detects memory leak for the attached example.
This doesn't happen when QTextDocument is used in the main thread.

Here is the detail:
==7558== 348,496 (720 direct, 347,776 indirect) bytes in 6 blocks are definitely lost in loss record 104 of 116
==7558== at 0x40277EE: operator new(unsigned) (in /usr/lib/valgrind/x86-linux/vgpreload_memcheck.so)
==7558== by 0x43FB1E5: QFontDatabase::load(QFontPrivate const*, int) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x43D156C: QFontPrivate::engineForScript(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x4410385: QTextEngine::fontEngine(QScriptItem const&, QFixed*, QFixed*, QFixed*) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44134B3: QTextEngine::shapeTextWithHarfbuzz(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x4414543: QTextEngine::shapeText(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44149BB: QTextEngine::shape(int) const (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x441D249: QTextLine::layout_helper(int) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x441E018: QTextLine::setLineWidth(double) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x445D8E4: QTextDocumentLayoutPrivate::layoutBlock(QTextBlock const&, int, QTextBlockFormat const&, QLayoutStruct*, int, int, QTextBlockFormat const*) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44639F6: QTextDocumentLayoutPrivate::layoutFlow(QTextFrame::iterator, QLayoutStruct*, int, int, QFixed) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)
==7558== by 0x44621FD: QTextDocumentLayoutPrivate::layoutFrame(QTextFrame*, int, int, QFixed, QFixed, QFixed) (in /usr/local/Trolltech/Qt-4.6.0/lib/libQtGui.so.4.6.0)