"Amazon SimpleDB provides a simple web services interface to create and store multiple data sets, query your data easily, and return the results."
SimpleDB sits on top of their hosted computing layer (EC2), proving an integrated cost model for those who are already EC2 customers. From a developer perspective, SimpleDB looks more like a big 'hashtable in the cloud' than a traditional RDBMS. The database is not relational and doesn't support SQL for data definition or data manipulation (DDL/DML). Instead, they have chosen to use a 'schemaless' system for data definition (name/value pairs) and for data manipulation (insert, update, delete) they have chosen to use simple REST style verbs (PUT, GET, DELETE, CREATE, QUERY).
The service is specifically designed for small payloads (not big BLOBS). For larger files, they recommend that you use AWS S3. They also recommend that you only run 'real time queries'. In fact, query execution time is limited to 5 seconds which prevent users from running very large requests. Although some might disagree with this decision, I'm a full supporter of this model. However, organizations will still need to perform two vital operations: 1. Export all data 2. Perform complex BI style queries. It is currently not clear how these items will be supported. The system doesn't seem to support the notion of 'change data capture' which would allow changed records to be sent to a separate analytics engine, nor am I finding a mechanism to easily load or unload a data store which would be a real issue if you time out after 5 seconds...
SimpleDB is in beta which implies that features have been frozen while bugs are ironed out. My primary concern is that they may have accidentally designed "TooSimpleDB". I love the idea of keeping it simple, but they may have overdone it.