Cloud computing: scalable DB

[David Bovill]

I'm looking into an application which needs a highly scaleable back end
database. One with low up front costs and yet won;t fall over if it suddenly
takes off. I've noticed Jerry and others describing On-Rev in terms of
"cloud" storage - but AFAIK it is a simple shared host, and there are no
services on offer with regard to scaling or true cloud computing?

So - I am looking at using true cloud based database solutions. On-Rev would
provide that administrative functions but web plugins on users own web sites
would directly call the cloud computing backend. I'm looking to prototype
this in revWeb and then implement the plugins in a suitable AJAX framework.
It seems to be that this is now a reasonable approach to take given the
existing state of these services - but I'm only reading about them and would
love some feedback from others on the list.

In particular I am looking at using Amazon - either their
SimpleDB<http://developer.amazonwebservices.com/connect/entry.jspa?externalID=2552&categoryID=146>,
or their Relational Database
Service<http://aws.amazon.com/rds/?ref_=pe_8050_15319810>,
but maybe also Google equivalent services? Anyone tried these - I've only
used Amazon S3 for file storage to date?