I really apologize for a very cursory read on my part, but I'm guessing it is in the same manner that CORS uploads to S3 is handled: http://aws.amazon.com/articles/1434/
If your application is only using S3 to upload objects, the full SDK would probably be overkill. Using pre-signed forms is sufficient there. However, if you want to use other services like DynamoDB, SQS, or SNS, pre-signed URLs will not work. Also note that in order to generate a pre-signed URL you still need a backend service running to sign those URLs, something you can avoid with the client-side SDK.
Ah ok yeah that makes sense. Pre-generate a signed token and post it along with the form. I guess in that case you wouldn't even need the jS-SDK since you can just do a normal POST/PUT to the s3 bucket with those credentials. I guess I got excited only to realize nothing would change ;)