I'm experiencing the same symptoms reported at https://bugs.launchpad.net/ubuntu/+source/xfce4-panel/+bug/977910 in a new Fedora 16 installation. Everything works fine for a few days, then I turn recover from sleep/suspend mode in the morning, and I can't switch between windows anymore using the buttons on the taskbar. They start flashing if I click on them. I can use alt-tab to switch between them, though.