I just wrote a post about mixing physics (chipmunk) with cocos2d (actually, ShinyCocos). Althought the code is ruby, it's really simple and I think it will be also useful for guy learning plain Cocos2D-iphone.
Here's the article:
http://rolando.cl/shiny_cocos/mixing_physics_and_sprites
Oh, one last thing: if you find any error in the post, please let me know :-)
Enjoy!
rolanod./