Reproduced, qtconfig says the default font is "Sans".

Dirk, can you give any input on what Qt4 is doing here?  Is "fc-match sans" the right thing to compare Qt's behaviour to?

Yes, "Sans" or "Sans Serif" is Qt should be the same as

"sans-serif" in fontconfig ("sans-serif" =  "sans serif" = "sans" in
fontconfig, there is no difference in these 3 spellings) 

I.e. when qtconfig says the default font is "Sans" it should
indeed used the font returned by "fc-match sans-serif".

This is due to hardcoded font substitution in Qt 4.4.3: "Sans Serif" is substituted by "helvetica" which in turn equals Nimbus Sans L according to what "fc-match helvetica" says.

This substitution is defined at src/gui/text/qfont.cpp on line 1768. After commenting out this line, Qt's "Sans Serif" on my system becomes equal Liberation Sans.

Corresponding patch for Qt 4.4.3:

diff -rup src/gui/text/qfont.cpp src/gui/text/qfont.cpp
--- qt-x11-opensource-src-4.4.3-orig/src/gui/text/qfont.cpp	2008-09-27 12:58:47.000000000 +0400
+++ qt-x11-opensource-src-4.4.3/src/gui/text/qfont.cpp	2008-12-21 02:04:55.000000000 +0300
@@ -1765,7 +1765,7 @@ static void initFontSubst()
         "arial",        "helvetica",
         "times new roman", "times",
         "courier new",  "courier",
-        "sans serif",   "helvetica",
+//        "sans serif",   "helvetica",  -- Better let fontconfig do it's work
 #elif defined(Q_WS_WIN)
         "times",        "times new roman",
         "courier",      "courier new",

Another option is changing /etc/fonts/conf.d/30-metric-aliases.conf

*** Bug 461181 has been marked as a duplicate of this bug. ***

Apparently this is the correct fix (see "Where are font mappings for generic fonts defined in KDE" thread on kde-devel@kde.org)

diff --git a/src/gui/text/qfont.cpp b/src/gui/text/qfont.cpp
index 7e7e9bc..d878d5b 100644
--- a/src/gui/text/qfont.cpp
+++ b/src/gui/text/qfont.cpp
 -35,6 +35,7 @@
 
 #ifdef Q_WS_X11
 #include "qx11info_x11.h"
+#include <private/qt_x11_p.h>
 #endif
 #ifdef Q_WS_QWS
 #include "qscreen_qws.h"
 -1756,6 +1757,10 @@ static void initFontSubst()
     Q_ASSERT(fontSubst != 0);
     if (!fontSubst->isEmpty())
         return;
+#if defined(Q_WS_X11) && !defined(QT_NO_FONTCONFIG)
+    if (X11->has_fontconfig)
+        return;
+#endif
 
     for (int i=0; initTbl[i] != 0; i += 2) {
         QStringList &list = (*fontSubst)[QString::fromLa

patch added to qt-copy and for 11.2, queueing for 11.1.