I seem to have 'fixed' it now but I wanted to report still more breakage in my Ubuntu 8.04 setup. The Last Straw was when the Fast User Switcher stopped detecting that I was already logged-in and started to reset my login each time. In other words, my wife would switch from my login to hers but then when I'd try to switch back it would crash all my programs.
The solution seems to have been removing the official Nvidia driver?! We are now running the very generic 'nv' (or is it 'nvidia'? who cares...) video driver.