What Open Transaction is trying to do is to provide a transfer mechanism, rather than issuing a currency, they use the word "cash" in this sense. If you want to hide your Bitcoin transfer, you can choose to use an OT server, which provably cannot see the link between two accounts involved in a transaction. As it's said in their FAQ, OT server merely provides receipt to be used as evidence, any redemption is between the users.
The difference with Ripple: it's open source, its author is very honest about everything(centralized as federated servers), it doesn't have a premined currency(no currency has a special status in the system), and it serves a purpose(anonymity).
EDIT: OP also proposed that, with Bitcoin's multi-sig escrowing, whenever an OT server signs something it can always be checked if it has enough reserve to back it up. to quote OP from here:
http://bitcoin.stackexchange.com/questions/517/how-can-a-single-person-operation-keep-a-collection-of-online-wallets-secure/834#834My proposal was, whenever she wants to bail back OUT again, she sends a signed request to the server, and then she forwards the server's signed reply to the members of the pool. IF the pool members have a record/audit of Alice's account (which must be in a DHT they share) and IF the two signatures both verify on the receipt, and IF the request is within the allocation for that server and its maximum bailout-per-day, then the pool members vote ON THE BLOCKCHAIN to release the funds back to Alice.
and:
s long as the pool members make audit information available to each other (preferably in real time), then they will always know whether/when a server exceeds the amount of funds in circulation than it actually has in the pool. An OT server cannot normally change your balance, or forge any of your transactions, since it cannot forge your signature on the receipt. But the server COULD potentially create a "dummy" account (that it controls) and then sign FALSE RECEIPTS with that account, thus inflating the currency. BUT--this could NOT escape notice of the afore-mentioned audit protocol.
So unlike a RIpple IOU, a open transaction cash token can be backed up.