I use jungledisk for backing up my photos ... it uses S3 for the backend. SE is cheap (about $1/month for the amount I've uploaded so far) and reliable.
Are you thinking of using a third party application for backups or rolling your own? S3 seems to have a pretty well developed API. I thought about writing my own application for the hell of it, but the amount of time needed to produce something reliable put me off that idea quickly)