I would like to offer you my MIT-licensed application, which includes projects and build targets for iPad, iPhone, iPod, iPad simulator, iPhone simulator, Mac OS X 32/64 bit fat, Windows 32-bit, and Windows 64-bit. It is fully cross platform and uses the Juce(*) open source library. The app demonstrates the application of audio filters (Digital Signal Processing), for which source code is also included:

"A Collection of Useful C++ Classes for Digital Signal Processing" by Vincent Falco
(Google Code)

Features include:
- MIT-Licensed(*), unlimited proprietary use!
- Single XCode 4 project with multiple platform targets
- Usage of child static library projects
- Usage of .xcconfig files to share build settings between projects
- Tuned build settings to run on the widest range of devices
- Turns off using Thumb for armv6 (performance increase)

In addition I have developed a short graphic tutorial for getting the most out of XCode 4 projects that target multiple devices:

XCode 4 Setup Tips and Tricks for iPad, iPhone, iPod, Mac OS
(Raw Material Software)

For those who can't be bothered to check out the repo via SVN, the latest source tree plus prebuilt binaries for both Mac OS X and Windows are available in a single .zip archive:

DSP Filters Project Downloads
(Google Code)

This fully functional project can be used as a starting point to develop your own cross platform app that will work everywhere!

(*) Juce cross platform application framework is separately licensed under the GPL, or a commercial license may be purchased from the author: Raw Material Software.