I wanted to give an update on the Mono support. As of version 1.2.5 of Mono (version soon to be released) we can say that Eiffel assemblies will work smoothly with Mono. Our last regression test for the compiler targeting Mono is only failing 20+ tests that the Microsoft .NET runtime is accepting (over 1350+ tests).
Until very recently, EiffelStudio generated .NET assemblies would not work with Mono. The culprit was the usage of the #- metadata table format (a non-standardized Microsoft .NET extension which allows specification of the metadata out-of-order) that Mono did not support.