From Amazon Web Services Email today:
Free Inbound Data Transfer (until June 30, 2010) Data Transfer into AWS will be free of charge from now through June 30, 2010, making it even easier for customers to get their data into AWS. This applies to data transfer into Amazon EC2, Amazon S3, Amazon SimpleDB, Amazon Relational Database Service, Amazon Simple Queue Service, and Amazon Virtual Private Cloud. Other applicable charges for use of these services continue to apply.
This is a good deal. Once your data is there, you can then do incremental backups and at minimal cost moving forward.