I had this same issue but in my case, as described on bug 65730, it was because the /apps/gnome-power-manager/action_button_suspend gconf key was set to nothing instead of suspend.
I have no idea how it got set to nothing, but I changed it to suspend and now it's working again on my T60. -- fn f4 key no longer generates suspend with feisty on thinkpad t60 https://launchpad.net/bugs/89141 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
