Very good writeup. This specifically covers setting up the firewall rules to only allow inter-group communication. This is very important to understand when working with a Riak cluster because Riak itself has very little to no internal security mechanics. You must proxy Riak through some middleware or somesuch for any real world usage.
Something to keep in mind is the multi-zone/geographic clustering. Brightbox specifically mentions they have a low latency link between their zones. I would test throughly. Riak is built on Erlang, which at its core is a message passing language. Riak by extension is very sensitive to the speed, throughput and latency by which those messages are passed amongst its members. Again, test throughly, YMMV.
