Enterprise includes a ring replication layer designed for higher-latency connections.
There is nothing preventing you setting up a cluster that spans continents. What will deter you is the poor performance of the cluster due to the added latency between nodes.
Interesting - from what I remember of the the original Amazon Dynamo paper it seems the ring replication is pretty central to the thing (if we are both talking about the ring replication used for the distribution of keys across the nodes).
This is sounding like crippleware :(
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.
Replication of keys around the ring is free. What you pay for is their solution to the problem of significant latency between nodes: code that replicates the whole ring in several sites and coordinates the communication between the sites.
The replication in the Enterprise version is replication between entirely different clusters. The ring replication you talk about is definitely open source.
There is nothing preventing you setting up a cluster that spans continents. What will deter you is the poor performance of the cluster due to the added latency between nodes.