Amazon has opened SimpleDB for beta testers. As you would expect the system is designed to scale and as such has dropped generic relational operations such as joins and transactions. What we're looking at here is another instantiation of megadata.
And before you start beating your chest and exclaiming how "real" applications need joins and transactions please read "The End of an Architectural Era (It's Time for a Complete Rewrite)" (pdf).
This paper builds on [SBC+07] by presenting evidence that the current architecture of RDBMSs is not even appropriate for business data processing.