I was just about to write up a little blog on the how and why of Exposing OData (Open Data Protocol) in WCF 4.0.
But seeming it being a Bank Holiday and, if you follow any of my posts, you'll know I don't spend a lot of time writing them up, they are more like mini blogs

So I found this excellent article by Brad Abrams

http://blogs.msdn.com/brada/archive/2010/03/16/silverlight-4-ria-services-ready-for-business-exposing-odata-services.aspx

I recommend you have a read because if you've never encounted OData before you'll be amazed how easy it is to expose and consume your domain service (expecially with Excel 2010)