Thanks to your reply, I got everything working. Apparently, the .dmg package was not appropriate. I used the sources instead. I took note of every step I followed, and I give those notes here.

1) Uninstall the inappropriate .dmg package :

sudo /Developer/Tools/uninstall-qt.py

2) Optional : Delete directories forgotten by uninstaller :

sudo rm -R /Developer/
sudo rm -R /Library/Frameworks/QtDesigner.framework

3) Download and install Xcode from Apple. A free registration is required :

http://developer.apple.com/technology/xcode.html

4) Download latest stable Qt from Public Repository :

http://qt.gitorious.org/qt/qt

5) Build Qt statically. Optional : Use -i to ignore errors :

./configure -static -debug-and-release -no-sql-sqlite
make -i
sudo make install -i

6) Install Qt Creator and specify in its preferences the Qt path.