As they just raised 100 million, I wonder if putting (a lot of) effort in replicating S3's functions to save part of 40K/month is worth the trouble. And setting that up won't be free either. (their 100 million can pay for the current storage for 200 years...)
No wonder 37signals decided to switch to their self-hosted storage solution[1].
[1] http://37signals.com/svn/posts/2483-nuts-bolts-storage