The pricing changes were announced on the AWS blog yesterday. The first tier of storage starts at $0.125 a month per GB for the first 1TB of storage, then pricing drops to $0.11 per GB/month up to 50TB, and so on. Note that there's no change in pricing past the 4,000TB+ tier, so really heavy users of S3 (like Dropbox) aren't really going to see a lot of pricing relief from the change.
According to Amazon, it's about a 12% reduction in costs for customers with about 50TB of data, and a 13.5% drop for customers with about 500TB. Note that there's no change to the request pricing, which is assessed based on the number of requests to work with objects in S3 storage. Data transfer prices are also unchanged.
This makes Amazon far cheaper than Rackspace, for example. Rackspace Cloud Files pricing starts at $0.15 per GB, and $0.18 per GB for outgoing bandwidth.
The price changes take effect as of February 1st, so customers will be getting the better pricing retroactively through the beginning of this month.