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At our site we generally prefer the key to the left of the "A" to be Control, and we don't need a Caps Lock key.
On existing x.org configurations we use 'Option "XkbOptions" "ctrl:nocaps"' to get this behaviour in X, which is usually good enough for us.
On Ubuntu 12.04 we have found this not to work. In addition to this, we have tried editing /etc/default/keyboard to contain 'XKBOPTIONS="ctrl:nocaps"'. This didn't appear to have an effect.
We have observed that when an individual user configures this behaviour through System Settings, ~/.gconf/desktop/gnome/peripherals/keyboard/kbd/%gconf.xml appears to record the configuration. However, even for admin users, we haven't located the "Apply For All Users" (or similar) option in the config application.
We would like to make Caps Lock act as a Control key by default for our users.
The most important requirements are that it works in X sessions started by GDM, and that individual users still have a way to override the setting for their own account.
Ideally, but less importantly, we would also favour solutions that work in all X sessions (not just those started by GDM), and also on the console.