When we announced Project Astoria at Mix 2007 last May we made the toolkit available along with a few samples as an online service. The missing piece there was the ability to create your own service online in the experimental setting we had setup. Now Mike has just announced the availability of the new version of the experimental online service that allows you to describe the schema of the data you'd like for your data service, and we'll create and host it for you.
So go ahead, create your data service and let us know how it works for you. To access the online service you can use HTTP directly or the client library that was included in the May 2007 CTP.