I don't get it. As long as an ARM binary or a 'Super-Secret A4 Architecture' binary is produced, who cares? I can write Objective C code in TextMate and then use xcodebuild from the command line to build a fully functional iPhone or iPad app.
Apple cares, because they don't want any significant portion of their application marketplace on the iThings to be controlled by any single API vendor, which would give that vendor market and PR leverage that Apple doesn't want them to have. It's not about compatability or looks or anything else...
Just think about the transition from PPC to INtel. For Xcode users - this was pretty much cake. The had to add some code in some cases, but it was a fairly easy task.
If, on the other hand, some other compiler run by someone else, hostile to apple, was the preferred platform for most apple developers, apple would have had a hell of a time pulling off a move like that.
If this was their goal, they could just mandate a specific C compiler. You can originally write in Scheme, translate it to C with Chicken and use whatever C compiler you want to compile the result. There would be no affect on any platform-switching plans Apple might have.
