With Ripple (and OpenTransactions) you are required to trust an "issuer" with the underlying asset. This is required for legacy assets like USD, but there is no technical reason that this is required for BTC. All of the benefits of Ripple can be had with the current blockchain and a properly designed, limited trust, network of Ripple or OpenTransactions servers.
By "limited trust" I mean that your assets are in your control at all times.
Let me explain in more detail:
Currently, any "value" in your account is an IOU from a trusted issuer. Say Alice Bank and Trust deals in USD. You can transfer USD into Alice's account. In exchange she will give you a USD IOU within the Ripple system. You must trust that Alice will safely store all of the USD she has been given. In theory, for every USD IOU that Alice has given out, there is 1 USD in their account. Anyone should be able to give Alice one of her IOUs and be given USD in return. If Alice is not careful with the underlying asset, all her IOUs can suddenly become worthless. If you deal in Alice issued USD IOU's in Ripple, you are trusting Alice will remain solvent and a good actor.
Currently, the same process is used for BTC in Ripple. Some issuer must give you an IOU that is theoretically backed by BTC. This is not required. Lets look at some examples of the advantages Ripple gives you:
Atomic transactions - You can transfer BTC IOU's for USD IOU's with no trust. This is incredibly useful if you want to build a decentralized exchange (judging by recent events, we definitely want one of those). You don't need to use BTC IOU's though. It is possible to transfer USD IOU's for raw BTC. You must risk your legacy assets with an issuer, you do not need to trust and issuer with your BTC. Think of the Ripple network as an alt-blockchain. We have strategies to do inter-chain atomic transfers (
https://bitcointalksearch.org/topic/p2ptradex-p2p-trading-between-cryptocurrencies-91843,
https://en.bitcoin.it/wiki/Contracts#Example_5:_Trading_across_chains).
There is no technical reason why this cannot be done within Ripple.
Instant transactions - Ripple can do instant transactions only because you trust that a small number of Ripple servers will never attempt a double spend. This is perfectly fine for old and well respected servers. Double spends are easy to prove after the fact, and the servers will work together to keep eachother honest. It would be exceedingly difficult to successfully perform a double spend, and if successful, that server will be banned for life and all the trust it had built will be lost.
However, we don't need to trust the Ripple servers (or issuers) with control of our assets to gain this ability. We only need to give the servers the authority to certify our transactions. This can be done with the block chain using multi-sig transactions. Create an account where, to spend the coins, your personal signature is required as well as the signature of the server. When you create a transaction, you send it to the server for certification. The server checks to make sure you have not tried to spend these coins before, and if everything checks out, it signs our transaction and broadcasts it to the bitcoin network. Any other bitcoin user can see this account is protected by the server and can choose to trust a transaction from this account with no confirmations. As long as the other user trusts the server to certify against double spends, accepting the transaction is safe.
The server can never steal your coins but, if the server disappears, you will be left unable to spend your coins. This is a better situation than we have now with trusting a Ripple issuer (if they disappear the IOU's are worthless as well). There are technical solutions to this problem that would require only minor additions to the bitcoin protocol.
Micro payments - Some types of micro transactions can be handled on the bitcoin network now (
https://en.bitcoin.it/wiki/Contracts#Example_7:_Rapidly-adjusted_.28micro.29payments_to_a_pre-determined_party). These are transactions where you have an ongoing relationship with the other party. This works if you will be spending between, say 0.05 BTC and 10 BTC, you just don't know exactly how much you will be spending at the beginning of the transaction. This does not work if you will be sending only 0.0001 BTC to a given user (say buying access to news article). A Ripple-like server network can help here as well by being an intermediary.
Lets say you are playing an MMORPG with an economy based on bitcoin. You commonly want to send very small amounts of bitcoin to different users. Each transaction may be 0.0001 BTC or so, but you tend to transact 0.1 BTC per week in the game. The workflow in the link won't work well because you want to setup a large number of small transactions with a number of different users. So, every player in the game sets up an account with the intermediary server. You reserve 0.5 BTC for possible payments throughout the week. The 0.5 BTC is not transfered to the server, it is just held in reserve so that you can't spend it elsewhere durring the week. When you make a transaction with another player, you increase your payment to the server by 0.0001 BTC and the server does the same with the other player.
In this scenario, you are trusting 0.0001 BTC to the server. They could take your coin and refuse to give it to the other player. However, this will be easy for you and the other player to spot and you both would stop doing business with the server. You would only trust the server with the largest micro transaction you conduct. By definition, I don't expect micro transaction theft would be a very lucrative business for a Ripple server.
In conclusion. Don't trust your BTC with an issuer. Build better tools. Ripple is great for lifting legacy assets into the modern bitcoin-like asset domain. That isn't needed for BTC, it already has all the features you need.