The ECL for iOS distribution includes a code example of a Common Lisp application running on the iPhone.

The example is a Swank server, the backend for the SLIME and MCLIDE Lisp development environments (disclosure: I am the developer of the latter).

Following the steps in the readme will build the application and place it on your iPhone/iPad, or alternatively on the iPhone simulator that comes with Xcode. You can then connect to the iOS device using SLIME or MCLIDE, providing a REPL from which you can evaluate Lisp expressions directly on the device and interactively develop the app.

The code example in the init.lisp file runs on the device as the application starts. It currently loads the swank server and notifies the user with a text field, button and alert dialog. The ECL for iPhone project doesn't yet provide much user interface functionality from Lisp, but you can use the Xcode Interface Builder to design the GUI.

ECL compiles to C and supports inline Objective-C, so you can use Common Lisp as an abstraction layer on top or integrate Common Lisp implemented functionality with Objective-C code. The repository for the project includes examples of combining Common Lisp with Objective-C, including the implementation of basic widgets.