I use Qt 4.8.5 configured with -arch x86 -arch x86_64 (so, x32/x64 binaries produced), compiled under Mac OS X 10.8 with XCode 4.6 (so, clang).
I need to recompile Qt toolkit myself because pre-build binaries are 64-bit only

Binaries are works OK:
in 64-bit mode: under Mac OS X 10.6 - 10.9
in 32-bit mode: under Mac OS X 10.8-10.9

Binaries does not work: under Mac OS X 10.6-10.7 in 32-bit mode any binary crashes in QApplicationPrivate::initializeWidgetPaletteHash() + 6394

To reproduce:

• build Qt under Mac OS X 10.8
• transfer any binary (e.g. Assistant.app) to Mac OS X 10.6 or 10.7
• Right click on app icon, Get Info, select 'Open in 32 bit mode'
• Try to run (crash report attached)

There is no problem if I build 32-bit version separately under 10.6.