I could see where that would be useful. Image uploads in our case are actually handled by our existing web app, so our image servers can be ignorant of user authentication/etc. We end up using swfupload, which is a nice client experience, and then in the callback we call directly into our image servers. Since it's an ajax experience, it's pretty nice, but if we had very large images, or if we had to pre-process out a number of different sizes/etc I could see where kicking off an async job would be useful.

It would be really cool to have an EC2 instance handle these resizes and put the images in new buckets for you - Might be a really neat implementation.