The roundtrip thing is an edge-case that doesn't really justify inflicting the non-deterministic pain on everyone. Python 3 and Java have taken the 'one true internal encoding' path and while hardly free of warts, it's an approach that is practically saner. The alternative is making some people's hell everyone's hell, forever.
"Hardly free of warts" doesn't even begin to cover the pain that's dealt with if you have to deal with these external encodings.
And, if you've got loads of data in an encoding that doesn't roundtrip, it's hardly an edge case.
Ruby's implementation is supposed to be such that if you want UTF-8 support and know that your (text) inputs and outputs are always going to be UTF-8, you never have to think anything differently than you did in Ruby 1.8. If it isn't working that way, then I think there's a bug.
