First off, I'm using XCode 4.0.2.
Okay, here is my issue. I can build Apple sample program for MFMailComposerViewController and run it in the simulator (I am aware it will not send email, I'm just concerned that it builds). That seems to work fine.
HOWEVER, when I try to build in my application I get the following message.
ld: warning: ignoring file /Users/lloyd/Projects/iOS/simpleApp/MessageUI.framework/MessageUI,
missing required architecture i386 in file
Undefined symbols for architecture i386: "_OBJC_CLASS_$_MFMailComposeViewController", referenced from:
objc-class-ref in MailData.o
ld: symbol(s) not found for architecture i386
collect2: ld returned 1 exit status
SOLUTION
Looking "Build Settings" (Basic and Combined) I saw that I had "Framework Search Path" set. This was odd because I never remembered setting that. Deleting it fixed the problem.
I am now able to build for both the simulator and the target. NOTE: You can NOT send email from the simulator, but you should still be able to run the code (if that makes sense).