In the last week, some of my menu icons (like 'Main Menu') had stopped working. Since they wouldn't launch, I couldn't determine what the problem was. Since the right-click didn't have a properties, I couldn't even determine what it was trying to do.
From a command line, run 'alacarte'. That complained about missing libraries for Python. Looking at the system, I could see that there was a recent upgrade from Python 2.4 to Python 2.5.
The correct solution was probably to import all of the libraries... but I didn't care about that. I had only upgrade Python in an effort to get some of the other apps to compile...
So I moved /usr/bin/python to /usr/bin/broken-python and re-symlinked /usr/bin/python2.4 to /usr/bin/python.
Now all the apps work again.