Have you (re)considered an option for a multi-threaded PUT now that Amazon can support this with multipart upload?

With this feature you can create parallel uploads…
(from http://aws.amazon.com/about-aws/whats-new/2010/11/10/Amazon-S3-Introducing-Multipart-Upload/ )

I found an old topic in this forum (from August 2009) which indicated you looked at a threaded solution once, but this was before Amazon supported parallel uploads.