The possibility of storing data in S3 is very nice. Unfortunately then you don't have the ability to user the security groups.
IMO: That would be perfect and inflict an order for a site license immediately (no joke). Maybe then you might 'worry' about concurrency. Splitting the read and write could solve that easily.
<em>edited by email@example.com on 3/22/2010</em>
You right about the concurrency, it's difficult to manage in a file. S3 is more useful for one person who want to get access to his connections from multiple places than a group of users. We are planning a solution for the end of year, but for now, there is no solution.