Check out this interesting post by Randy George concerning S3 map tiles for DeepEarth:
The project also includes an example showing how to set up a local tile set. The example uses 256×256 tiles but not in the OSGeo TMS directory structure. Here is an example using this DeepEarth local example TileProvider DeepEarth BlueMarble. You can see the tiles spin in from a local directory store on the server. The resolution is not all that great, but a full resolution BlueMarble isn’t that hard to get from BitTorrent. The alternative selection “Blue Marble Web” tiles are full 500m resolution hosted on Amazon S3 courtesy of ModestMaps.org. The Amazon S3 bucket is a flat structure, in other words the buckets don’t have an internal directory tree, which is why the tiles are not stored in a TMS directory tree.
The DeepEarth local TileProvider was easily adapted to suit the TMS directory so I could then directly pull in my El Paso tiles and show them with a DeepEarth interface. However, if I wished to take advantage of the high availability low latency S3 storage, I would need to flatten the tile tree. In S3 subdirectory trees are hiddeen inside buckets as file names. In the S3 case the tile names include the zoom level as a prefix: http://s3.amazonaws.com/com.modestmaps.bluemarble/5-r8-c24.jpg The 5 on the 5-r8-c24 nomenclature is the zoom level while row is 8 and column 24 at that level. TMS would encode this in a tree like this: ../bluemarble/5/24/8.jpg. The zoom level= subdirectory, row = subdirectory name, and column = name of tile. The beauty of TileProvider class in DeepEarth is that minor modifications can adapt a new TileProvider class to either of these encoding approaches.
Performance and reliability is a lot nicer on an Amazon S3 delivery, especially with heavy use. Once in S3 a map could also be promoted to a CloudFront edge cache without much difficulty. I imagine that would only make sense for the upper heavily used zoom levels, say 1-5 for BlueMarble. Once down in the level 6 part of the pyramid the number of tiles start escalating dramatically and repetitive hit rates are less likely.
Update (2009-03-20): It's pointed out in comments that I'm wrong about the obstruction, and that you can in fact simulate the proposed WMTS tile resource hierarchy in S3. I'm not wrong about the RPC-ness of the candidate spec, something that can yet be fixed.