As the article mentions, these are all built around the SheevaPlug (http://plugcomputer.org/). I guess there is enough of a market that these are becoming more and more popular to build and sell. My idea with these is basically a local+cloud storage hybrid: you attach a local drive via USB, but then connect it to AWS and use that as the main storage area. Then the local drive is simply a very large cache. The appeal is that you get "infinite" storage with the speed of a fast NAS drive.
It would be pretty trivial to get Fuse - http://code.google.com/p/s3fs/wiki/FuseOverAmazon - installed on the SheevaPlug, which would give you a mountable "directory" to your S3 account which you could then share on your local network.