There are other possibilities, too.  Bitcoin could serve as a master currency that underwrites other currencies issued against it.  For example, you could use Open Transactions to issue a digital currency backed by Bitcoin holdings, which would be publicly-verifiable through inspection of the blockchain.  Any transactions in this new currency wouldn't have to touch the Bitcoin network or blockchain.  The same is true for Ripple--you can establish credit and conduct commerce based on Bitcoins without actually burdening the Bitcoin network for transactions.  As an imperfect analogy, Bitcoin could become to pseudonymous currencies what CHIPS is to the traditional banking system.