Just installed 12.04 desktop and I was astonished to discover no easy way to open an xterm or gterm or kterm (a shell prompt). Writing lengthy shell scripts is a lot easier in a shell. :)
I thought Ctrl-Alt-T opened gnome-terminal by default? I believe this also works in Lubuntu.
For Unity, you can hit the Super or 'windows' key and type 'term' or some variation on it to get several terminal options (depending on what you've installed).
In Unity you can also easily make a shortcut. Hit super/windows key to open dash, type 'short' or 'shortcut' and it takes you to the keyboard shortcuts with a couple of mouse clicks.
Alt+F2 works in several desktop/windows environments. In some versions of openbox or blackbox, you can hit F12 to open a terminal.
Hit Alt+F2 and type gnome-terminal
(Then install ccsm and terminator and set up a hotkey. I use flag+R :)