There are a couple of approaches that Cadillac has taken to keyless ignitions.
My favorite and the current approach, is that the Cadillac recognizes that you have the key fob in your pocket or purse, you press the brake pedal, and press this button:
To turn off the car you simply press the button again.
The previous incarnation of this system has a traditional ignition tumbler with a sort of permanent thick key head in place that one can turn as if a traditional key. I suspect that system was quick and easy to apply, since it only required putting a different ignition tumbler in and not a different dash panel, button, harness, etc.
The button is much better in terms of how keyless should work, and more fun. The 2006-2009 Cadillac STS-V had this kind of button, and some 2010 Cadillacs do.
Keyless ignition is generally available only on fully optioned models. Hopefully all future Cadillacs with keyless will have the button on the dash; it is a much more elegant solution than the previous fix.