Being able to create a unique device specific presentation layer is important, while still being able to iterate business logic quickly in c# - as it can be shared cross platform iPhone/Android/Windows phone.
Although, there are a number of possible issues I see arising. These would be painful down the road if any were to occur, I have listed few but there are others:
Apple could revert their decision to accept applications with third party frameworks into the app store.
Monotouch/Monodroid loses the support of Attachmate, or Attachmate changes its support of in a way in which future iterations of the app are in jeopardy.
Please note, that I have no issue in migrating the project to objective-c. There would be a number of benefits to switching