To open Layout option, click the button near the bottom leftThe layout Options window has some related settings:
Default options for Caps Lock behaviour doesn't allow it to be disabled (before Ubuntu 10.04)Caps lock should now function as a Compose key (which need to be held with multiple other keys to have an effect) and stop interfering with your typing if you accidentally hit it. (I actually find this better than completely disabling it)
Unfortunately, all of the options just change an accidental press's side-effects. (The options for Ubuntu 10.04 includes an otion to disable Caps lock) Luckily, another option exist.... The Compose key has no side-effects when accidentally pressed and might actually be useful at times... Ubuntu allows you to set the Caps Lack key to act as a Compose key. To find this option, open the "Compose key position" option and select Caps Lock. Caps Lock can be configured to function as a compose key
Update: This works under Ubuntu 10.04 as well. In addition, Ubuntu 10.04 has a "Caps Lock is disabled" option in the second dialog.