Software Sources requires GTK 2 and won't run on 11.04

I had the same problem and finally solved it! Just for the record, reinstalling the packages as stated above did not help. This is what I did.

According to the pygi docs, if you type

python -c 'from gi.repository import Gtk; print Gtk'

you should get

<gi.module.DynamicModule 'Gtk' from '/usr/lib/girepository-1.0/Gtk-3.0.typelib'>

(Gtk-2.0 depending on which version you have installed. I have Gtk-2.0.)

This gave me the clue. The Gtk typelib was in fact there but I suspected there may be another girepository-1.0 directory in my filesystem that was taking precedence. Using the locate command I found it in /usr/local/lib along with several libgirepository-1.0* libs. This directory did not contain the Gtk typelib. So I simply removed it along with the libraries and the problem is now solved.

Since software-properties is a Python application, it actually depends on package gir1.2-gtk-2.0. If you don't have this package installed, you have broken dependencies. Check it out using Synaptic.

Otherwise reinstallation of the affected packages may help:

sudo apt-get install --reinstall gir1.2-gtk-2.0 python-software-properties software-properties-gtk