It's a menu option, under settings. Don't see a menu?? :
I'm running Ubuntu 12.04,
One thing I notice is that the menu (File, Help, & such) does not seem to display if the program is shut down, and then restarted. If the window is maximized, these items show up again.
You are not the first to report strangeness with Unity (the bastard pile of MacOS wannabe move-the-menu-to-the-top bs that Ubuntu slapped on Gnome in order to keep up with Windows 8 in the stupidifying of GUIs). There is an official issue here:
https://github.com/bitcoin/bitcoin/issues/1242 where you can add your steps to replicate on 0.8.0.
The workaround is
sudo apt-get install kubuntu-desktop.