At Mix 07 we announced the Astoria Data Service which offers a REST based access to relational data.  At that time we posted several static databases on live.com.  Yesterday we enabled anyone to push their own data to http://astoria.mslivelabs.com via an experimental service. 

You can easily access Astoria Data Service from any Ajax application (via the easy URL format), a Silverlight application (via the Astoria Silverlight 1.1 client library) and of course any .NET applications

Read all about it on the Astoria Blog.