Yes, that's what I am thinking: a week of backups to S3 and a script that does moves the oldest one from S3 to Glacier. You would rarely need a backup older than a week if you've already got a week of backups. I'm still figuring out if there is a practical way to do this with incremental backups without introducing to much risk that the backup process gets messed up.
"In the coming months, Amazon Simple Storage Service (Amazon S3) plans to introduce an option that will allow you to seamlessly move data between Amazon S3 and Amazon Glacier using data lifecycle policies."
In a true disaster recovery (Building burned down or is otherwise unavailable) - it usually takes most businesses a week or so just to find new office facilities.
But - agreed, there will be some customers for whom Glacier wouldn't work well for all use cases.
Now - a blended S3/Glacier offering might be very attractive.