Ask HN: S3/Self-hosted/Eucalyptus? Help us optimize bandwidth for big app launch

HNers, here's hoping we can get some advice on what to do with a looming bandwidth bill... We are about to roll out an app that will have 1 million people using it in a matter of days. We're sure of this because of the nature of the application and the previous experiences of our client. So, our app must download 7-10MB of data to 'boot'. 10MB x 1MM users = 10TB of data. An equivalent amount of data will need to be downloaded about once/month. Using S3, we're going to have a pretty big bandwidth bill, around $2500/month. Hosting our own, we could get to $50/TB using Ripple Web and we could use Eucalyptus to remain S3-compatible - $500 + server = $800? Anyone else moving away from S3 because they're trying to optimize for bandwidth? Anyone else optimizing for bandwidth costs in creative ways? Are there really cheap CDNs out there that we should consider?