How do I write a program that can alter what character is produced when a key is struck even if the program is not running?

If that's not possible, how do I make it work even if the application doesn't have the focus?

On that note, how are keyboard layouts made? That's not exactly what I'm trying to do, but if I knew how it would solve my problem.

Thanks!!

I don't really have any experience with that and after some brief google searching I couldn't find many results. You might try some of the Apple mailing lists, the cocoa one specifically.