CFM is the old format used since System 7.1 (whenever PPC support was first introduced), and then abandoned in OS X. OS X will still read CFM PPC apps, but you can't produce a Universal Binary using CFM, and Apple's tools can't be used to create a CFM app on OS X.
Oh, and there is the whole issue that you then have to use the old resources stored in the resource fork, meaning you have to call into the Carbon APIs, you can't use Interface Builder, and localization is by and far, MUCH more time-consuming.
Yeah... uhm... I wouldn't use CFM. You essentially will bind the life-span of the app to that of PPC Macs, and forcing Intel systems to run in emulation just to use the app.
Other than that... there is no way to hide the contents of your bundle from the world. The real question is: What the in the world are you attempting to hide from the end user that you would need to do this?