This comment is amusing, since Cocoa runs/ran across an amazing number of platforms. NeXT shipped the API's on 680x0, x86, SPARC, HP PA-RISC, etc. You may want to check out www.gnustep.org as well. Even better, the development tools allow you to build an application once and deploy it natively on every platform. Wonderful stuff. Apple ships Coca on numerous platforms as WebObjects, though (oddly) they don't allow application vendors to license them, so even though Carbon app's run across a half-dozen or so platforms, you can't really sell them that way. Still, it runs great on MacOS X, or anywhere WebObjects is installed. I sure wish that Apple would allow software vendows to license the runtime.
Of course, the MacOS API's, including Carbon, are implemented in a completely Mac-hardware specific manner, so they'll never run on anything else (unless Apple throws $millions at porting it). So while it might be the case that procedural API's might be mappable ( though if you get into the details, the MacOS API's and Windows API's are quite different in almost every detail aside from a few cross-ported technologies (e.g. TrueType, QuickTime).
So what it comes down to is that developers will have to pick between Carbon and Cocoa. And, having used both, Cocoa is clearly the right way to go unless you have an overwhelming pile of legacy code, and even then you're simply trading off a quick port now against a long, losing battle against competition using Cocoa and getting 10x your productivity. So any smart company, even if they're porting a legacy Mac app to Carbon had better be exploring a Cocoa port.