MonoTouch library projects should have a MinOSVersion value. This would fix a couple of problems:
a) MonoDevelop could pass this value to ibtool when compiling nibs into the dll (via IBC_MINIMUM_COMPATIBILITY_VERSION), preventing a crash when running on older OSes.
b) MonoDevelop could embed the value in an assembly attribute in the dll, then when building an exe referencing that dll MD could verify that the executable's MinOSVersion value was no older than the dll's MinOSVersion value. This would be especially useful with third party dlls and binding dlls.
Additional attributes that would be useful:
* Min version of Xamarin.iOS (useful for binding projects, which may require a certain version of Xamarin.iOS to work properly).
* Build version of Xamarin.iOS.