When talking to some customers and partners, there was some confusion regarding regions and high availability. There is a assumption that if you use regions you will not get High Availability. That is not correct – High Availability is orthogonal to whether you put your keys within a region or not.
The way it works is that we map keys and keys within regions to partitions. The partition is nothing but a a range of hash values. (see my MIX talk) . All the high Availability and load balancing etc. is done at the partition level. So whether you put your keys within a region or not you will get HA.