On the assumptions that:
1) The cost of data transfer from customers to your servers is low
2) The marginal cost of adding storage to your servers is low
Why not offer a backup/data storage service? This could simply be more space available on a customers webspace, perhaps accessed by WebDAV or rsync.
Could you beat Amazon S3's US$0.15/GB/Month?
Offer a Backup Service
Have a read through this thread for some comments and discussion on this topic.