$0.15 per GB-Month of storage used. $0.20 per GB of data transferred.
So, that means Dropbox is going to have to resell S3 at a premium for the added value of these nice Coda-like features. Would you pay a premium for these Dropbox features? Maybe, I don't know.
Also, what's the typical use case? How much bandwidth/storage are people going to consume? Because, if I store 100 megabytes... my bill will pennies every month (going on S3 prices). You cannot transact pennies per user per month. If you could, then you've cracked the micropayments problem wide open. Maybe there would be a base fee? Like $5/month or something. Would people pay that much for online storage?
I don't know how other revenue generation models can be applied. Advertising? Selling a user's data to other businesses? (What's the privacy policy?)
More interesting is the user experience. Creating something users can enjoy, agree with, and possibly part money for is a much more difficult problem to solve than figuring out to make large scale storage cost effective.