Replicas (or as you put it, ring replication) is critical, and Riak very much has replicas. What it doesn't have in it's open source version is multi-ring replication (cross DC) which is a separate concern.

In the Dynamo paper the ring spans DCs but they also have a very different network than most that allows them to do that. In Riak it is recommended that each ring is contained in a single DC. If you want ring-to-ring replication from Basho then you can pay for Riak EDS. You could also build it yourself as others have mentioned (Kresten Krab Thorup has done something like this in Riak Mobile [1]).

Nothing is stopping you from running a single ring (cluster) across DCs, and it might even be okay for certain apps, but it's not a choice that should be taken lightly. In general, if you don't understand the tradeoffs you're making in that regard then it's best to stick to one ring, one DC.

[1]: http://www.erlang-factory.com/upload/presentations/413/Erlan...