Hi, I can confirm the transparency problem. It works on Ubuntu's default Unity 7, though.
Also, transparency works under both Mutter and Unity 7 with some other emulators based on the same VTE, namely gnome-terminal with Ubuntu's transparency patch, Terminix, and xfce4-terminal. Forwarded upstream: https://bugs.launchpad.net/terminator/+bug/1647052 e.