I've gone down that path before of storing encrypted secrets in SimpleDB. Unfortunately, encrypting string values will bloat the length of your strings, bumping against the 1024 value limit of SimpleDB: http://docs.aws.amazon.com/AmazonSimpleDB/latest/DeveloperGu...
Didn't think about that. There are, of course, ugly workarounds, but I just asked about SimpleDB not because it's fun to use, but because of its low cost in this scenario.