I'm running with a Radeon HD 6400M, and 13.04 seems to really not like the updated graphics drivers (or at least, that's the conclusion I've come to). After installing them I had exactly the same issue.
For me, the following worked:
sudo apt-get purge fglrx
followed by (possibly after a reboot, I can't quite remember. Probably do one just to be sure)
gsettings reset org.compiz.core:/org/compiz/profiles/unity/plugins/core/ active-plugins
The latter command seemed to take quite a long time to run, but once it had everything worked fine thereafter. Hope this helps.