Today, Basho announced the release of a Riak AMI on the AWS Marketplace. From the Basho blog:
A number of our community members and customers already use Riak on AWS, and with the Riak AMI getting up and running should be much easier. At its simplest, the AMI is a ready-built virtual machine image and configuration of Riak for those who use Amazon EC2.
You can pick it up on the AWS marketplace here.