I added comments on GNOME bugzilla, but I have more comments relevant to just Ubuntu.
When this locks up, there is almost no way out for me. Ctrl+Alt+Backspace hasn't worked in a long time, so that's not an option. Ctrl+Alt+1 (or 2-6) works, but because of some other bug (NVIDIA, perhaps) the VT is invisible. (I also get a sort of "backlight bleed" effect sometimes.) The VT is present so I can log in and `killall gedit`, but I have to do it blindly. There's also no sshd by default, so I couldn't even log in remotely to kill the process.
There needs to be a way for a user who doesn't know these tricks to get back to working system without pulling the power.