Is it possible to reverse-engineer my iPhone application?

On OSX an Application Bundle is just a directory named .app with some special meta-data files about the application, and the application executables and its resources. Right clicking and picking Show Package Contents will show the entire contents of the Application.app directory.

Any file resources, images, sounds, properties filed, etc. that are not using some application specific encryption will be plainly accesible.

That said, the application code, if a native Objective-C application, will be compiled and not easily reverse engineered, it would have to be decompiled, and if stripped of debugging information would take a dedicated hacker to grok the assembly output and modify it.