Correct me if I'm wrong, but it's still possible to double spend the transcripted coin?
1. Alice sends Bob the using the off-chain system.
2. She also redeems it onto the chain. She can do this anytime before Bob does (or whoever he passes it on to).
If I understand correctly, the transaction that redeems the coin back into the Bitcoin chain embeds inside it a required SCIP proof of a transcript that specifies that the latest history of this coin is that it is now spent back to the Bitcoin network. Thefore, Alice cannot also send this coin to Bob in the off-chain system, under the assumption that the double-spending prevention in the off-chain system is functional. But we'll wait for more info from gmaxwell...