I just upgraded to 13.04 from 12.10 in a 64-bit Parallels VM, and the semi-transparent terminal windows I configured for Gnome Terminal are now totally opaque.
I'm setting transparency via Edit -> Profile Preferences -> Background -> Transparent background. This is also illustrated in http://www.ubuntugeek.com/howto-create-a-transparent-terminal-in-ubuntu-desktop.html. Adjusting the slider has no effect.
Any workarounds?