All of the previous answers involve having the upload pass through your node.js server which is inefficient and unnecessary. Your node server does not have to handle the bandwidth or processing of uploaded files whatsoever because Amazon S3 allows uploads direct from the browser.
Have a look at this blog post: http://blog.tcs.de/post-file-to-s3-using-node/
I have not tried the code listed there, but having looked over it, it appears solid and I will be attempting an implementation of it shortly ad will update this answer with my findings.
