hmm.. you are using 2.6 and im using 2.5

Maybe you could try installing the 2.5 package to see if it helps?

apt-get install python2.5-qt4-gui

Edit: Scratch that, I don't think it would make any difference... hmm
Im afraid I dont know enough to help you out with this one..

If you just type "python" at the n900 terminal, do you get the python info returned? You might just need reinstall the pyqt package again...