We're actively working on supporting Silverlight, and refactoring much of our underlying Entity and caching structure to better support it. This includes removing the DataSet and our reliance on the NetDataContractSerializer. Some of this support will be trickling into the product in the next beta, and true Silverlight support is planned for a June time frame.
That said, I'm not sure how extensible the BOS will be right away regarding exposing additional endpoints. You should, though, be able to do what's supported in DevForce v3 today to expose your business objects via your own web service.