I both wouldn't bet on Joyent's Smartmachines being a magnitude cheaper and being as fast:
On the speed: We run a Riak cluster, and for us the actual response times we get from Riak are, as described, slower than what Amazon promises in its docs.
On the price: If you would get 3x16 GB machines with Joyent, that would cost you 1400$/month. You can get a lot of resources for that with this new AWS service.
I don't have any experience with either Joyent or this new DynamoDB, but I do have some experience with Riak, and from the docs this new service would be a very viable competitor.