This is enough if all the nodes remain online and they are all well-behaved, but in case not, I've proposed a system of commit registries as an extension that should be fairly robust against node failure and cheating:
https://groups.google.com/forum/#!msg/rippleusers/Ruy_QIb0AAY/fUF5bGNJWXkJ
I think that is similar to the suggestion to use the bitcoin chain itself as register.
You promise to pay subject to the bitcoin payment actually happening before a certain block number. The bitcoin block chain can also be used to add arbitrary messages into the block chain, though this is discouraged. These are effectively permanently timestamped as part of the bitcoin transfer system.
That means that each ripple transaction has to be matched by a bitcoin transaction.
For small payments, it should be possible to only reset the chain every few ripple transactions. This prevents the bitcoin chain being used to certify that the main transaction took place.