Mono is a .NET Framework for Linux, you can load Linux on the ISeries.
Ergo you can run .NET on the ISeries.
I have not tried the above, but would think it should work.
Of course if IBM would implement the .NET Framework with the same passion as
they the ISeries JVM, then we would be talking a real serious machine ;-)
(note I already consider the AS/400 - ISeries to be a serious machine! the
framework would 'kick it up a notch'.)