Amazon Glacier is like Amazon S3 except that it’s much cheaper to store data (1 cent vs 13 cents per GB per month, uploading still for free) and accessing that data takes much longer (hours!). It will be interesting to see whether backup applications such as Arq will able to take advantage of Glacier (probably in combination with S3).
Update (2012-08-22): sintaks:
While the link between your device and the service will be much fatter, the reason Glacier is so cheap is because of the custom hardware. They’ve optimized for low-power, low-speed, which will lead to increased cost savings due to both energy savings and increased drive life. I’m not sure how much detail I can go into, but I will say that they’ve contracted a major hardware manufacturer to create custom low-RPM (and therefore low-power) hard drives that can programmatically be spun down. These custom HDs are put in custom racks with custom logic boards all designed to be very low-power. The upper limit of how much I/O they can perform is surprisingly low - only so many drives can be spun up to full speed on a given rack.
Retrieval requests are priced differently, too. You can retrieve up to 5% of your average monthly storage, pro-rated daily, for free each month. Beyond that, you are charged a retrieval fee starting at $0.01 per Gigabyte (see the pricing page for details). So for data that you’ll need to retrieve in greater volume more frequently, S3 may be a more cost-effective service.