Just read this article over at CNET concerning Apple blocking some apps from the iPhone AppStore, for having "duplicate functionality" with respect to built-in iPhone Apps. This is too bad to hear.  The built-in apps aren't revenue generating for Apple, so i'm not sure what the reasoning behind this is. If someone comes up with a better Calendar manager, or better Mail program, why not let consumers choose, just like on the Mac platform. 

[update]

here's another good article on this subject from the NY Times.