A simple but interesting demo of using Cocos2d with Box2D physics can be found at http://www.vellios.com/2010/06/06/box2d-and-radial-gravity-code/
The Objective-C code using Cocos2d can be found at that link.
Converting it to Cocos2d-x and C++ was very easy and provides another good example of porting existing Cocos2d code to various platforms using C++.
Most of the code on this blog can also be deployed using the native code from Cocos2d-x or Marmalade SDK for those that are targeting multiple platforms. Just look for the include “mkb” file.
The only part about this post that was tricky was doing the Box2d debug draw but luckily the code is included in the Box2d test code in Cocos2d-x. The extra files required were GLES-Render.cpp and GLES-Render.h and they are included.
The source code can be found at: http://www.neonchalk.com/files/RadialGravity.zip
You will need to have the cocos2d-x development environment. Load the cocos2d-win32.vc2008.sln (for example) and just add the RadialGravity.win32.vcproj to the solution.
The code has been updated and tested with cocos2d-1.01-x-0.11.0 release that includes and updated version of Box2D.