I've seen a few post here in the past about using Amazon S3 servers with membership sites.
The obvious problem with this is that if you make your files public on S3, anybody can access them, if they know the url.
If you make your files private, your members cannot see them!
This is a problem I faced recently, but with a lucky find of a post in the War Room, I found the answer (just search on Amazon S3 in the War Room). The War room is about $20 to join, but just to find that post it is worth the membership cost many times over.
The post is by Kyle Tully and I must publically thank Kyle for his help in getting me up and running.
The basic premise of Kyle's post (and underlying script) is that you set your files up as private in S3. You then host a php script on your webserver which passes your ID and Secret ID to S3.
I'm not the most technical person, and this was a stretch for me, but I got there in the end, and I can now sleep well at night knowing that I'm probably on the best server in the world (possibly the cheapest!), and that is is secure.
If you have the same challenge as I had, them subscribe to the War Room and look up Kyle's excellent post.
P.S. I also discovered how to upload to S3 using the new firebox browser and the S3 plugin. This too is excellent, and worthy of another post!