The : command is often used in shell scripts to trap and ignore signals received by the script.


trap : 1 2 3 9 15

will cause the shell ignore any signal 1, 2, 3, or 15 sent to it while executing commands in the script.
Sorry, Nobody can ignore a signal 9.
Re: ":" and vi. Technically, ":" is an escape from the visual front-end of vi out to the underlying editing program, ex. Typing a : inside vi allows you to execute any ex command. This escape key is a colon because ex's traditional prompt is a colon.