I'm the co-founder of Avocarrot (http://www.avocarrot.com/) and we were wondering for a while whether to build a dedicated plugin for Xamarin. How easily can you integrate an native Android or iOS SDK with your Xamarin apps? Would it make a big difference for you if a Xamarin plugin was available?
1. You can bind the native iOS or Android library into C#, which is relatively straightforward:
http://developer.xamarin.com/guides/ios/advanced_topics/bind...
http://developer.xamarin.com/guides/android/advanced_topics/...(.jar)/
2. Or you could create a single, managed library in C#, which would also make it easier to get Windows support.
Option 2 is more work.