The ASP.NET team has built in a really extensible provider model [1] into the "building block" services in ASP.NET 2.0. Here are some postings related to this topic: Jit Ghosh demonstrate [2] how to build a custom provider for the Membership system. Daniel Cazzulino [3] discuss some pitfalls in the implementation that certainly should be fixed, esp. this: "...maybe my provider uses a webservice...". Nice thought, why not. Andy Smith takes a deeper look on the provider model. Steven Smith [5] explains the breaking changes in VS2005 March Preview. I had this problem already preparing my slides for my talk on asp.net 2.0 last week.
[1] http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnaspnet/html/asp02182004.asp
[2] http://msdn.microsoft.com/asp.net/default.aspx?pull=/library/en-us/dnaspp/html/bucupro.asp
[3] http://weblogs.asp.net/cazzu/archive/2004/03/22/93838.aspx
[4] http://weblogs.asp.net/asmith/archive/2004/04/13/112076.aspx
[5] http://blogs.aspadvice.com/ssmith/archive/2004/04/03/918.aspx