Yup, that's it. It's failing to find FSharp.Core.sigdata and FSharp.Core.optdata; if you copy them to /usr/lib/mono/gac/FSharp.Core/FSharp.Core/4.3.0.0__b03f5f7f11d50a3a/ you should find that fsharpi works. I clearly had those lying around from a previous dirty install.
I think we'll need to teach some mono tooling about F#.
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