I had this problem also and here's what happened in my case. The python libraries did not seem to install correctly when I upgraded to diablo. The main offender being python2.5-gnome. I think it relates to the fact that the diablo repos were not populated when I first installed diablo so the libs came from the chinook repos instead.

To verify if this is the problem that others are having, run mClock from xterm. Or run python and type 'import gnome.gconf'. When it fails you may see a message like this "No module named gconf".

To fix this, I uninstalled python2.5-gnome which required removing the python2.5-runtime also. Then I installed youamp because it is in the repo and has a dependency on gnome so it will force a proper reinstall of gnome. Then the mClock install should work again.