Are you a startup who need a cheap database, with guaranteed backup and uptime? Well, you'll love Amazon latest software-as-a-service project called SimpleDB. This is a bigger and better version of their popular S3 service... but instead of just storing and retrieving files, you can also QUERY your data storage.
What's great about it, is that you only pay for what you use. Upload a terabyte into the system, transfer a terabyte out, and it will cost you around $1800 per month. If you are a startup who needs Amazon quality backups, recovery, uptime, and a terabyte of database storage, I highly doubt there's a cheaper option... except for perhaps placing the less structured data into S3... if all you need is a hashtable lookup to find a file, or a message queue, SimpleDB might be overkill.
The big question is... what exactly is it??? Its too simple to be a "real" database, and since everything is stored at Amazon.com, performance tuning will be tricky to say the least. Marcelo Calbucci has offered that SimpleDB is actually a directory service, and NOT a database.
I think a really killer way to do this is to use S3 and SimpleDB as a persistence layer for Oracle Coherence. That combo would let you keep a local cache of your Amazon database... so access would be a lot faster. Also, a little known fact is that Coherence can store Java code as well as data. This means invoking Java code in Coherence will enable executing the code in the same process-space as your raw data... which means incredibly fast performance.
Coherence on top of S3/SimpleDB is an application cloud on top of a data cloud. Coherence for speed and code execution, Amazon for cheap and nearly infinite scalability. That sounds pretty damn cool to me...
Of course, such a solution is mainly for startups, small, and maybe mid sized companies... I doubt most enterprise customers would be kosher with storing data at Amazon. Firstly, enterprise customers have already spent a lot on their data centers, and have made them pretty cost effective... even if their centers are more pricey than Amazon, its a big political hot potato.
Secondly, there's the security issue. Who wants to store their data at Amazon? Of course, if you could do an encryption step in Coherence before storing data to Amazon, then some of those issues might be moot. Of course, substring queries on encrypted data is pretty impossible, so you need to be careful.
Anyway, cool stuff. I hope someday to get to use it ;-)