Just in case anybody want's to give Amazon's new (currently in closed beta) SimpleDB web service a test drive with Rails...
I just finished work on a Proxy Server bridging the APIs. It's available as a Rails Plugin:
http://inside.glnetworks.de/2008/01/20/bridging-rails-to-amazon-simpledb-using-activeresource/ (also available via http://agilewebdevelopment.com/plugins/aws_sdb_proxy )
Cheers, Martin Rehfedl