Handling Wordpress media files on EC2/S3 with auto scaling
You might consider looking at something like s3fs (http://code.google.com/p/s3fs/). This allow you to mount your S3 bucket as a volume on your server instances. You could simply have the code to mount the volume executed on instance start-up.
s3fs also has the ability to use local (ephermal) directories as a cache to the s3fs directory so as to improve performance.