Google today released Google App Engine SDK 1.1.6 (announcement, download) which, among other things, brings with it datastore keys that you can sort and filter on.
Your keys can have names but they cannot start with a number. Keys can also have IDs, which are numeric. We can read those, but can't set them.
When I put an entity into the datastore for the first time, you assign it a numeric ID. I'd love to be able to create an entity on a different instance with the same ID you've assigned it but I can't. I'd love to be able to create an entity with the same key that you've assigned it (again, on a separate instance -- say the local SDK or a different app), but I can't set keys directly.