I need to get a list of tasks for a user from a WCF service to display on an iPhone app. I'm planning to use something like http://www.nayyeri.net/custom-username-and-password-authentication-in-wcf-3-5 to authenticate, but how would I pick up the username/user ID to filter my list of items?
You'll need to follow the example in the answer to this question on custom validator access to the user name. This good overview of custom service credentials shows how all the pieces work together. On the iPhone side, you'll need to configure the client to perform Basic HTTP authentication over HTTPS to successfully call the service.