Amazon has announced a new online backup service that seems to be well suited to our needs.
At $0.01 per GB / month Amazon Glacier is 10x cheaper than Amazon S3! For instance, storing 500GB of data in the cloud would cost $60 per year instead of $600.
From Amazon: "Amazon S3 is a durable, secure, simple, and fast storage service designed to make web-scale computing easier for developers. Use Amazon S3 if you need low latency or frequent access to your data. Use Amazon Glacier if low storage cost is paramount, your data is rarely retrieved, and data retrieval times of several hours are acceptable."
The problem is there is currently no GUI for Glacier but I'm sure it won't be long. (Jungle Disk currently only support S3.)http://aws.amazon.com/glacier/