One of the useful things about S3 for content storage is that a client can directly make a direct HTTP request to download the object. For instance, this is how Twitter serve up avatar images.

SimpleDB provides an HTTP interface to data. Rather than having to write a proxy that sits inbetween SimpleDB and the client, is it possible for client software (i.e. desktop, mobile) to make calls to read values from a SimpleDB domain, without sharing credentials that shouldn't be shared? Or is a proxy in-between the only way to go?