Amazon’s SimpleDB isn’t a relational database, but it does provide elastic scalability and high-availability. Amazon’s recently announced Relational Database Services (RDS) is a relational database, but it doesn’t provide elastic scalability or high-availability. If you are deploying enterprise applications on the cloud (including Amazon Web Services), you might want to look at ScaleDB because it is a relational database and it does provide elastic scalability and high-availability.
Amazon describes SimpleDB by comparing it to a clustered database:
"A traditional, clustered relational database requires a sizable upfront capital outlay, is complex to design, and often requires extensive and repetitive database administration. Amazon SimpleDB is dramatically simpler, requiring no schema, automatically indexing your data and providing a simple API for storage and access.