Hi AJ,
I have taken a quick look at the AWS SDK but have not yet looked into it deeply enough to give you an informed opinion on how it compares with JetS3t.
What is completely obvious is that the AWS SDK offers the benefit of supporting many different services in a fairly consistent way, whereas JetS3t supports only S3 and CloudFront.
Due to JetS3t's focus on S3 and its long history my feeling is that it still offers a much richer feature set than the AWS SDK when interacting with S3. However I would be interested to hear others' views, so please follow up here when you have had a closer look.
I do still intend to support JetS3t but I am not interested in competing with Amazon's library: I support JetS3t because people find it useful and it meets a clear need. I assume the AWS SDK will become richer over time, which will mean that the need for JetS3t (and my support of it) will decrease. So basically I expect my library to gradually fade into a well-earned retirement.
However, I don't have a good idea for the timeframe over which this is likely to happen. That will depend largely on JetS3t users, so let me know what you think.
Cheers,
James