I've read over the agreements several times, and I can't find anything that guarantees Amazon will not lose or corrupt your data. Do they make any such guarantees? Can I trust S3 as my long term backup storage?
See Amazon's terms, specifically, section 11.5:
WE AND OUR LICENSORS DO NOT WARRANT THAT THE SERVICE OFFERINGS WILL FUNCTION AS DESCRIBED, WILL BE UNINTERRUPTED OR ERROR FREE, OR FREE OF HARMFUL COMPONENTS, OR THAT THE DATA YOU STORE WITHIN THE SERVICE OFFERINGS WILL BE SECURE OR NOT OTHERWISE LOST OR DAMAGED.