Just a quick note to say that although I really struggled to find a way to get multipart uploading working with S3, I eventually found a patch that works for me at https://github.com/Adys/s3cmd/blob/master/S3/S3.py

The adys-threaded-multipart branch of code at https://github.com/s3tools/ is as far as I can see a earlier version of this code, and does not fully work (although it is fairly easy to hack it to get it to work).

I have tested this code fairly heavily over the last week or so and its working well for me in production. Thank you to Jeremy for creating such a useful patch, and I hope it is merged soon!