QMessageBox on Mac doesn't include the app icon with its Standard Icon

Product
Qt
Function
Painting
Version
4.6.0-tp
Platform
Mac OS X
Platform details
Mac OS X 10.5.8, also tested on Mac OS X 10.6 - same failure.
Compilers
GCC
Compiler details
Subject
QMessageBox doesn't include the app icon with its StandardIcon
Steps to reproduce / test case
run the examples/standarddialogs code, that comes with qt.

in Qt4.5.2, the critical dialog shows the larger yellow triangle, and has the application icon overlaid on top of this, scaled down, at the bottom right - this is correct in terms of the Mac OS X look & feel guidelines - and mimics correctly the behaviour of cocoa applications that use the NSAlert API.

with the other error styles: Information, Question and Warning - the Qt4.5.2 system displays only the application icon with no overlay (also correct).

The bugs in Qt4.6 are:

• no application icon is shown in the QMessageBox dialog
• no yellow exclamation point with app icon overlay is shown with the QMessageBox::Critical style
• Warning, Information and Question styles show an icon in the QMessageBox
  More information
  I've attached a cocoa app showing the different styles, pretty similar to the StandardDialog example code in Qt.