I thought I might pass along information on mpicc integration into Xcode (I currently use v2.3 and have not tested this on other versions). Xcode has a compiler specification interface that -- according to what ever internet websites exist on .pbcompspec files -- has not been released to the public. Needless to say, one nights worth of rooting around in xcode compiler specification files led me to try this simple interface file. Which, amazingly enough, works on my system (Mac OS 10.4.8). Attached is the file "MPICC.pbcompspec" that should be placed in the "/Library/Application Support/Apple/Developer Tools/Specifications" directory. Upon starting Xcode this file is loaded and added to the list of compilers it knows about.
To use the mpicc compiler, open your project, get info on your target, click the rules tab, and add a new entry. Change the process rule for "C source files" and select using "MPICC". No more custom build scripts or changing linking flags with new mpi versions :)
A few caveats. In the file, the ExecPath parameter should be set to the location of your mpicc install. A little more complicated issue is that the BasedOn parameter should -- probably -- refer to the gcc compiler version that mpicc calls. (My system uses gcc-4.0.)
I hope this helps anyone who's interested.