I am building a web service for my ASP.NET MVC 3 website. Ideally I would like all clients (my web pages, iphone application, android application, windows phone 7 application, e
This would be a great opportunity for you to try out WCF Data services, together with Entity Framework. You can build it very fast, and it exposes your information model using the OData international standard in both XML and JSON flavor. The interface is accessible throught REST. No problems interacting with any front-end technology, fat, web or mobile clients.
AND, you have all the benefits of re-using your C# .NET knowledge, visual studio and created assemblies to also make more tighter integration with Windows applications if you need to. It certainly did the trick for me!
So:
You can find a walkthrough at: http://msdn.microsoft.com/en-us/magazine/ee336128.aspx
Happy coding!