Tarsnap does block-based differentials, in addition to its fancy crypto footwork. You can't set that up with bash, truecrypt, and S3.
But I have no doubt the reason while your revenue doesn't make you happy yet it is because hackers are not the right target for it. I do differential backup using encFS and dropbox; it's not even remotely as cool as tarsnap but it's free for now. You see my point?