I posed the same question not too long ago. Of the three cars I've owned before my IS, I've always used Engine Bright by Gunk. With that stuff, you simply cut the engine off and wait until it's cool, hose it down, spray on the Engine Bright, let it sit for a few minutes, hose it off, and then let the engine dry. It's always worked great without any problems. But I've also never had a car that had as much electronics under the hood than the IS.
I'm thinking about having my IS engine steam cleaned somewhere instead.