I think something to strongly consider is Apple's motivation.
I agree with other sentiments posted online that Apple is trying to prevent commoditization of applications - that is to say, having more and more applications written using frameworks that generate applications that can run across multiple devices.
But that's not what Monotouch is. Monotouch is all about using the Apple frameworks to write applications - but through Mono, not Objective-C. So from that standpoint what Monotouch is doing is not something that should really bother Apple.
I still hold that developers are better off writing in the native language of the platform they are using, as things are just generally smoother when you don't introduce system that can have abstraction impedance mismatch - the Cocoa frameworks were all built to be used from Objective-C, and they make the most sense when you are used to the philosophy of Objective-C. But I do hope that Apple comes down on the side of allowing MonoTouch to be used.
