Building kdebase 4 with current gcc 4.3 snapshots causes several important applications (e.g. plasma, konsole) to crash on startup. Several files are involved; rebuilding the workspace/libs/plasma directory with an older gcc 4.3 snapshot (or gcc 4.2.x) fixes plasma startup. Last known good gcc 4.3 snapshot: SVN rev. 129944 No simple testcase found yet.
I've run checks on some more gcc revisions. Down to a few revisions. Last known working svn rev: 130324 First known broken svn rev: 130330
A Qt strict-aliasing bug potentially affecting this has shown up; it's possible that the problem isn't with gcc after all, but only shows up with the current versions. Running more tests with a fixed Qt version. (At a first quick look, I don't see how 130324->130330 could un-hide a strict-aliasing bug though, especially because the strict-aliasing bug is supposed to occur with 4.2.1 as well)
Waiting until the submitter determines whether or not this is actually a compiler bug.
This was identified as a Qt bug by us (there's a dup with a testcase around somewhere which I cannot find right now).
FWIW Dirk's fix for this taken from OpenSuse is available at http://svn.pardus.org.tr/pardus/devel/programming/libs/qt4/files/qt-43-aliasing.diff
*** This bug has been marked as a duplicate of 34385 ***