Interesting, reading that makes it sound like a C# developer that wants to work with MonoTouch will have to verge off to learning at least some Objective C if they want to have shippable code.
The closest analogy to C# develop would be COM and P/Invoke. C# provides tools to interface with COM and native libraries, but you still need to learn some marshalling fundamentals so you can ensure GC doesn't collect objects while they are being used by native code, you need to implement IDisposable and not forget to free unmanaged resources, etc.
It's similar with MonoTouch: you need to understand that MonoTouch classes “wrap” native objects, that disposing of wrappers doesn't necessarily kill native objects because other native objects may still link to it, and that sometimes GC can't know for sure if an object is eligible for killing, and will never collect it.
In general, this comes down to: don't rely on .NET GC to kill native objects, do it yourself. My life got so much better after I started doing so.
Things that confused me most (and which are all you really need to know about ObjC memory management):
http://stackoverflow.com/questions/13058521/is-this-a-bug-in...
http://stackoverflow.com/questions/13064669/why-cant-monotou...
This is a great screencast I wish I saw before I started coding in MonoTouch:
http://forums.xamarin.com/discussion/33/finding-memory-leaks...
It explains how to find native memory leaks.
How much time does it take to learn this? No more than a week. But it takes more to find memory leaks in existing app so it's better to get some understanding and learn to profile early. For me, “fixing” a two-month-old codebase took about week and a half.